Stars are earned by achieving a pre-set score on a level. 

The higher the score, the more Stars you win, up to a maximum of three per level.

They can be used to unlock gates. If you don't have enough stars, replay levels where you haven't achieved all three Stars.

On mobile, check your star total in the bottom left-hand corner of the map.

On Facebook, check your star total at the top right-hand corner of the map.

Note: Your highest score on each level can differ across platforms causing your Star total on each platform to differ.