Meaning of this passage
The purpose of this passage is a call, a warning, to avoid apostasy. This passage is very clearly saying that people who have received God's blessings should not turn away from God and that if they do, they will never be able to again receive God's blessings.
This passage is a call to avoid apostasy.
The one who falls away isn't one who sins, but one who turns completely from God and his beliefs: the apostate. The warning here is against someone who completely rejects God and his Son.
No doubt, any sin whatsoever turns us away from God; any sin is a falling away from Christianity. But the warning here isn't against sinning, but from turning away from the grace of Christianity—renouncing God, his Son, and the grace offered.
For he falls away who forsakes the word of God, who extinguishes its
light, who deprives himself of the taste of the heavens or gift, who
relinquishes the participation of the Spirit. Now this is wholly to
renounce God. —John Calvin, Commentary on Hebrews
Brought Back to Repentance
This passage speaks a very dire warning to avoid turning away from God—to avoid not only sin, but apostasy. That if we do such a horrible deed, it will be impossible to be "brought back to repentance".
In a commentary by Charlene McCaa,
The warning of this passage is to be not like Esau, who held the things of God lightly. That one sold his birthright as the first born son for temporal gain or satisfaction, then sought repentance with bitter tears, but to no avail. What was his, then given up, was lost forever. —Charlene McCaa, Hebrews Commentary Project
We see that when someone turns away from God and his grace, lays aside his salvation. This person cannot take it up again. Attempting to return to God would be like crucifying Christ again:
Crucify Christ Again
But the reason why he says, that Christ would thus be crucified again, is, because we die with him for the very purpose of living afterwards a new life; when therefore any return as it were unto death, they have need of another sacrifice —John Calvin, Commentary on Hebrews
Loss of Salvation?
This is a battleground topic.
Some believe that loss of salvation is possible. To those people, this is one of the passages that clearly illustrates the point.
To those who believe that loss of salvation is not possible (Perseverance of the Saints), this is viewed as passage that is speaking to the unbelievers—to the non-Christians:
But I cannot admit that all this is any reason why he should not grant
the reprobate also some taste of his grace, why he should not irradiate
their minds with some sparks of his light, why he should not give them
some perception of his goodness, and in some sort engrave his word on
their hearts... There is therefore some knowledge even in the reprobate,
which afterwards vanishes away. —John Calvin, Commentary on Hebrews
From Calvin's commentary, we can see that this is a warning for those who are non-believers, yet have tasted God's grace. From this perspective, this passage is not about loss of salvation (since the reprobate had no salvation to lose), but rather about the gifts that come with knowing God's grace.
This passage can be clearly understood to mean that if we have accepted God's grace, we should not turn from it. If we do turn from that grace and reject God, we will never be able again to enter into that grace.
Whether this leads to loss of salvation, however, is a doctrinal issue. This passage does not necessarily provide concrete proof for loss of salvation. It entirely depends on the doctrinal stance from which you approach the passage.