Achievements are gameplay reloaded goals that can be completed as you progress forward in the game, unlock new areas, and pass the levels in those areas. As soon as a goal conditions are met, you will receive a reward. 

You can check what kind of achievements are available in each area by tapping the Map picture and then Trophy on the left side of the game screen. Tap the blue and white arrows next to the area names to see the achievement goals for an area.