When you open the game, you will see a horizontal bar with a chest on the main screen, below your balance of hearts and coins. To get it, you will have to complete a certain number of levels.
As you complete levels, the bar will fill up; once you reach the required number of levels, the chest will parachute down and you will receive a prize when you click on it.
You will also have the chance to upgrade your prize in exchange for watching a promotional video.