Lives represent the number of turns you can play. You will see the number of lives you have in the upper right corner of your screen, these being represented by a heart.
When you start the game you will have 10 lives; every time you start a game in any game mode or lose your turn, you will have used a life that will be deducted from your balance.
For more information on these items, we invite you to visit the article: How can I get more lives?