Hearts are used in Challenge Mode, you need them to be able to move forward in that mode.
The amount of Hearts you have will be displayed on the top left corner of the challenge mode menu.
As you can see, this user had 5 hearts at the time
Hearts are spent each time you play and take 3 hours to reset each unit, you can get hearts by:
-Waiting the allotted time for them to be refilled
-Asking your friends to send them to your inbox
-Buying them in our shop
Should you choose the later this is the screen you will get
You can get other currencies, for other thing so make sure to check the article about our In Game Currencies