What is the bonus pool in Starcraft 2?

Solution 1:

To extend Raven's answer this sc2 faq has a good description:

Q. What is the bonus pool and how are bonus points acquired? A. The Bonus Pool is an accumulation of points that every player receives whether they're online and playing or not. They're essentially used as a means to help give a player a catch-up boost if they haven't played in a while. The pool does have a cap but it increases slowly until the end of a season.

Q. How is the bonus pool used in competitive play? A. When you achieve victory against another player, you are awarded a certain number of points. The Bonus Pool points will match the number of points you received for winning.

For example: If you were to have a Bonus Pool of 30 and you win a match that would normally give you 10 points, points will then be pulled from your Bonus Pool to match those 10 points and you'll be awarded 20 points for winning that game. Your bonus pool will then sit at 20.

Solution 2:

The bonus pool encourages players to keep playing ranked games and improve their ranking. It makes the matchmaking non zero-sum, which in turn means its not a viable strategy to get to 1st on the ladder, and then sit there for 3 months while no one can approach your zenith.

In actual terms of mechanics, it is a bonus value added to your rank when you win. Extra earned rating will match, point for point, any rating you accrue from winning games, until the pool is empty.

It fills over time whether you are playing or not.