What does season lock exactly mean in Starcraft 2?

Season Lock fixes your League. You can not be promoted or demoted, but you still earn points within your Division. Bonus pool also stops accumulating for all players.

Your hidden matchmaking rating (MMR) is still adjusting unrestrained, though. If your MMR stabilizes within another League during Season Lock, you will be promoted/demoted immediately after the one qualification match in the next season.

Season Lock is intended to introduce "home stretch" feeling in the end of a season. Because bonus pool stops growing, players who played rarely can catch up on those who used up their bonus pool. League placement Feats of Strength are awarded at season end, so there is an added reason to inch a few places in the Division to gain, for example, a "Top 25" instead of "Top 50".

Season Lock means the League is locked yes, meaning you cannot be demoted/promoted, but you gain/lose Points within your League.