You stamp a calander in the game every day you play the game. Get enough stamps and you unlock more minigames. If you do well enough on a certain minigame you can unlock an extra difficulty level.
This is a short review, but Brain Age 2 is a short game. Its a lot of fun working every day to unlock all the games and difficulties, but when you do theres not much left for you to do and after a few hours of playing you'll almost certainly be very very bored.
Brain Age 2 isn't a bad game at all, but I would have loved to see more depth and replayability. Hopefully when Brain Age 3 comes around it will reach the full potential of smart making minigames.