The number of lives (hearts) indicate the number of attempts you have to complete a level. You start the game with the maximum number of five lives. You can find the number of lives you have by checking the heart icon at the top of the home page. 

Each time you fail to complete a level, you lose one life. One life is replenished every 30 minutes, so you can quickly return to having 5 lives 🙂

Certain events can award you with infinite lives for a certain amount of time. This means you will not be losing lives in that duration no matter how many times you fail.