Lives refer to the number of attempts or chances you have to complete a level. 

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. You can also purchase a life package with your coins!

You can find the number of lives you have by checking the heart icon at the top of the home page.

Watching an advertisement will get you one free life if you prefer not to wait. Still, there are several offerings in the store that provide unlimited lives for a limited period of time.