Susan gave a great answer focused on the greek word itself - allow me to supplement it with how this passage and that greek word has been understood over time in Christianity. Here are some of the core historical references/commentaries:
Helvidius (~383 AD) gives the argument that that Matthew 1:25 is evidence against the perpetual virginity of Mary.
Helvidius (quoted by Jerome in Against Helvidius):
The passage for discussion now is, [Matthew 1:24,25] "And Joseph arose from his sleep, and did as the angel of the Lord commanded him, and took unto him his wife and knew her not till she had brought forth a son, and he called his name Jesus." [...] the adverb till implies a fixed and definite time, and when that is fulfilled, he says the event takes place which previously did not take place, as in the case before us, "and knew her not till she had brought forth a son." It is clear, says he, that she was known after she brought forth, and that that knowledge was only delayed by her engendering a son...
Why could not Scripture say, as it said of Thamar and Judah, [Genesis 38:26] And he took his wife, and knew her again no more'? Could not Matthew find words to express his meaning? He knew her not,' he says, until she brought forth a son.' He did then, after her delivery, know her, whom he had refrained from knowing until she was delivered."
Jerome (~383 AD) responds to Helvidius' argument, arguing that Matthew 1:25 is not evidence against Mary's Perpetual Virginity.
Jerome replies to Helvidius in Against Helvidius:
Our reply is briefly this,--the words knew and till in the language of Holy Scripture are capable of a double meaning. As to the former, he himself gave us a dissertation to show that it must be referred to sexual intercourse, and no one doubts that it is often used of the knowledge of the understanding, as, for instance, "the boy Jesus tarried behind in Jerusalem, and his parents knew it not." Now we have to prove that just as in the one case he has followed the usage of Scripture, so with regard to the word till he is utterly refuted by the authority of the same Scripture, which often denotes by its use a fixed time (he himself told us so), frequently time without limitation, as when God by the mouth of the prophet says to certain persons, [Isaiah 46:4] "Even to old age I am he." Will He cease to be God when they have grown old? And the Savior in the Gospel tells the Apostles, [Matthew 28:20] "Lo, I am with you always, even unto the end of the world." Will the Lord then after the end of the world has come forsake His disciples, and at the very time when seated on twelve thrones they are to judge the twelve tribes of Israel will they be bereft of the company of their Lord? Again Paul the Apostle writing to the Corinthians [1 Corinthians 15:23-25] says, "Christ the first-fruits, afterward they that are Christ's, at his coming. Then cometh the end, when he shall have delivered up the kingdom to God, even the Father, when he shall have put down all rule, and all authority and power. For he must reign, till he hath put all enemies under his feet." Granted that the passage relates to our Lord's human nature, we do not deny that the words are spoken of Him who endured the cross and is commanded to sit afterwards on the right hand. What does he mean then by saying, "for he must reign, till he hath put all enemies under his feet"? Is the Lord to reign only until His enemies begin to be under His feet, and once they are under His feet will He cease to reign? Of course His reign will then commence in its fullness when His enemies begin to be under His feet. David also in the fourth Song of Ascents [Psalm 123:2] speaks thus, "Behold, as the eyes of servants look unto the hand of their master, as the eyes of a maiden unto the hand of her mistress, so our eyes look unto the Lord our God, until he have mercy upon us." Will the prophet, then, look unto the Lord until he obtain mercy, and when mercy is obtained will he turn his eyes down to the ground? although elsewhere he says, [Psalm 119:123] "Mine eyes fail for thy salvation, and for the word of thy righteousness." I could accumulate countless instances of this usage, and cover the verbosity of our assailant with a cloud of proofs; I shall, however, add only a few, and leave the reader to discover like ones for himself.
The word of God says in Genesis, [Genesis 35:4, Septuagint] "And they gave unto Jacob all the strange gods which were in their hand, and the rings which were in their ears; and Jacob hid them under the oak which was by Shechem, and lost them until this day." Likewise at the end of Deuteronomy, [Deuteronomy 34:5-6] "So Moses the servant of the Lord died there in the land of Moab, according to the word of the Lord. And he buried him in the valley, in the land of Moab over against Beth-peor: but no man knoweth of his sepulchre unto this day." We must certainly understand by this day the time of the composition of the history, whether you prefer the view that Moses was the author of the Pentateuch or that Ezra re-edited it. In either case I make no objection. The question now is whether the words unto this day are to be referred to the time of publishing or writing the books, and if so it is for him to show, now that so many years have rolled away since that day, that either the idols hidden beneath the oak have been found, or the grave of Moses discovered; for he obstinately maintains that what does not happen so long as the point of time indicated by until and unto has not been attained, begins to be when that point has been reached. He would do well to pay heed to the idiom of Holy Scripture, and understand with us, (it was here he stuck in the mud) that some things which might seem ambiguous if not expressed are plainly intimated, while others are left to the exercise of our intellect. For if, while the event was still fresh in memory and men were living who had seen Moses, it was possible for his grave to be unknown, much more may this be the case after the lapse of so many ages. And in the same way must we interpret what we are told concerning Joseph. The Evangelist pointed out a circumstance which might have given rise to some scandal, namely, that Mary was not known by her husband until she was delivered, and he did so that we might be the more certain that she from whom Joseph refrained while there was room to doubt the import of the vision was not known after her delivery.
Martin Luther (~1539 AD) agrees with Jerome against Helvidius - and goes even further, stating that Matthew 1:25 is evidence for the Perpetual Virginity of Mary.
Martin Luther, in "That Jesus Christ was Born a Jew":
Now this refutes also the false interpretation which some have drawn from the words of Matthew, where he says, "Before they came together she was found to be with child." They interpret this as though the evangelist meant to say, "Later she came together with Joseph like any other wife and lay with him, but before this occurred she was with child apart from Joseph," etc. Again, when he says, "And Joseph knew her not until she brought forth her first-born son" [Matt. 1:25], they interpret it as though the evangelist meant to say that he knew her, but not before she had brought forth her first-born son. This was the view of Helvidius which was refuted by Jerome.
Such carnal interpretations miss the meaning and purpose of the evangelist. As we have said, the evangelist, like the prophet Isaiah, wishes to set before our eyes this mighty wonder, and point out what an unheard-of thing it is for a maiden to be with child before her husband brings her home and lies with her; and further, that he does not know her carnally until she first has a son, which she should have had after first having been known by him. Thus, the words of the evangelist do not refer to anything that occurred after the birth, but only to what took place before it. For the prophet and the evangelist, and St. Paul as well, do not treat of this virgin beyond the point where they have from her that fruit for whose sake she is a virgin and everything else. After the child is born they dismiss the mother and speak not about her, what became of her, but only about her offspring. Therefore, one cannot from these words [Matt. 1:18, 25] conclude that Mary, after the birth of Christ, became a wife in the usual sense; it is therefore neither to be asserted nor believed. All the words are merely indicative of the marvelous fact that she was with child and gave birth before she had lain with a man.
The form of expression used by Matthew is the common idiom, as if I were to say, "Pharaoh believed not Moses, until he was drowned in the Red Sea." Here it does not follow that Pharaoh believed later, after he had drowned; on the contrary, it means that he never did believe. Similarly when Matthew [1:25] says that Joseph did not know Mary carnally until she had brought forth her son, it does not follow that he knew her subsequently; on the contrary, it means that he never did know her. Again, the Red Sea overwhelmed Pharaoh before he got across. Here too it does not follow that Pharaoh got across later, after the Red Sea had overwhelmed him, but rather that he did not get across at all. In like manner, when Matthew [1:18] says, "She was found to be with child before they came together," it does not follow that Mary subsequently lay with Joseph, but rather that she did not lie with him.
Elsewhere in Scripture the same manner of speech is employed. Psalm 110 reads, "God says to my Lord: 'Sit at my right hand, till I make your enemies your footstool.'" Here it does not follow that Christ does not continue to sit there after his enemies are placed beneath his feet. Again, in Genesis 28, "I will not leave you until I have done all that of which I have spoken to you." Here God did not leave him after the fulfillment had taken place. Again, in Isaiah 42, "He shall not be sad, nor troublesome, till he has established justice in the earth." There are many more similar expression, so that this babble of Helvidius is without justification; in addition, he has neither noticed nor paid any attention to either Scripture or the common idiom.
John Calvin (~1562 AD) also agreed with Jerome in his commentary on Matthew 1:25 that this passage was not evidence against Mary's Perpetual Virginity.
John Calvin, Works "Volume 31, Harmony of the Evangelists" #25, page 107:'
And knew her not. This passage afforded the pretext for great disturbances, which were introduced into the Church, at a former period, by Helvidius. The inference he drew from it was, that Mary remained a virgin no longer than till her first birth, and that afterwards she had other children by her husband. Jerome, on the other hand, earnestly and copiously defended Mary's perpetual virginity. Let us rest satisfied with this, that no just and well-grounded inference can be drawn from these words of the Evangelist, as to what took place after the birth of Christ... It is said that Joseph knew her not till she had brought forth her first-born son: but this is limited to that very time. What took place afterwards, the historian does not inform us. Such is well known to have been the practice of inspired writers. Certainly, no man will ever raise a question on this subject, except from curiosity; and no man will obstinately keep up the argument, except from an extreme fondness for disputation.
Above references taken from the article Who were the brothers of Christ?