Lives represent the maximum amount of attempts you have to beat levels. When you use up all of your lives, it takes some time to refill them.
You can have a maximum of five lives, and you lose one every time you lose a level. It takes 30 minutes to refill one life. If you don't want to wait, you can use diamonds 💎 to refill lives or ask your friends for lives by tapping the heart icon ❤️ in the top left corner of the screen.
IMPORTANT NOTE! If a friend sends you an extra life when you already have five, the maximum amount, you won't be able to use it. But don't worry, it'll become available once you lose one of your current lives.