Locking is stops any further interaction. A locked post can't receive votes or comments. A closed post can. Only moderators can lock or unlock a post.
Things usually only get locked if we've closed them and they continue to attract bad comments or votes. It's just a good way of keeping things clean. It's one step short of deleting the question entirely, which we try to avoid so the poster has some reference to see why something was closed and know what to do next in the case of a duplicate.
