First, let's answer this part of your question, what is the meaning of this:
And whosoever lieth carnally with a woman, that is a bondmaid,
betrothed to an husband, and not at all redeemed, nor freedom given
The woman is a slave, and is in that sense not free, i.e. not redeemed out of her enslaved status as a bondswoman. She is, however, engaged.
Next, why weren't both the slave woman and the man who had carnal knowledge of her executed? They are not executed because she is a slave, and penance can be made as described in Leviticus 19:21-22.
There is ambiguity in the translation of "scourged" from the original Hebrew. To be scourged means "to be whipped as a punishment." The KJV of Leviticus 19:20 phrases it as:
she shall be scourged; they shall not be put to death, because she was
Other versions, e.g. ESV, of that passage read as follows:
If a man lies sexually with a woman who is a slave, assigned to
another man and not yet ransomed or given her freedom, a distinction
shall be made. They shall not be put to death, because she was not
Instead of "scourged", the Hebrew word used could mean "investigated", see here
The original Hebrew text reads בקרת תהיה, “there shall be biqoreth”.
This last word is variously translated as “investigation” or
“punishment”, but it seems only the KJV applies this specifically to
the woman. The Hebrew text doesn’t support this at all, so it’s
unclear why the KJV translates the text this way. Perhaps this was a
mistake; perhaps they had a variant text reading בקרת תהיה לה, “there
shall be biqoreth to her”;1
Not only is the woman not scourged, but neither she nor the man are put to death.2 Instead, the man must do the following, see the rest of the passage from Leviticus 19:21 through 19:22 (KJV) 3
21 And he shall bring his trespass offering unto the LORD, unto the
door of the tabernacle of the congregation, even a ram for a trespass
offering. 22 And the priest shall make an atonement for him with the
ram of the trespass offering before the LORD for his sin which he hath
done: and the sin which he hath done shall be forgiven him.
Here is a clearer exposition, from a non-StackExchange source! If the woman were not a slave, then both the woman and the man would have been executed.
If a man has carnal relations with a woman who is a slave and has been
designated for another man, but has not been redeemed or given her
freedom, there shall be an indemnity; they shall not, however, be put
to death, since she has not been freed. 4
By making this offering, the man attones 5 for his sin:6
But he must bring to the entrance of the Tent of Meeting, as his guilt
offering to the LORD, a ram of guilt offering. With the ram of guilt
offering the priest shall make expiation for him before the LORD for
the sin that he committed; and the sin that he committed will be