Achievements are long-term tasks that you can complete in order to earn cash!
Each achievement has 3 steps and you will be rewarded with cash each time you complete a step. You'll see an exclamation point on the achievements button or Eva will remind you to accept your achievement gift when you complete one.
You can always check your achievements from the Achievements Building (the giant golden cup!) if you want to!
Pro Tip: The Expansion Master achievement can be done by expanding your cafe to the last 3 expansions which are 23x23, 24x24 and 25x25.