What does Galatians 3:18 mean?

Brothers, speaking of human terms, though it is only a man’s covenant, yet when it has been confirmed, no one makes it void, or adds to it.

Now the promises were spoken to Abraham and to his offspring. He doesn’t say, “To descendants”, as of many, but as of one, “To your offspring”, which is Christ.

Now I say this: A covenant confirmed beforehand by God in Christ, the law, which came four hundred thirty years after, does not annul, so as to make the promise of no effect.

Then why is there the law? It was added because of transgressions, until the offspring should come to whom the promise has been made. It was ordained through angels by the hand of a mediator.

Now a mediator is not between one, but God is one.

Is the law then against the promises of God? Certainly not! For if there had been a law given which could make alive, most certainly righteousness would have been of the law.