All players with the free version of the game have lives. You need lives to start battles with opponents. Starting a game costs one life. If you win, you get the life back; if you lose, you don’t. You automatically regain the maximum number of lives over time. You can regain lives faster by upgrading your Library or by equipping an item that restores lives faster. You can also get your lives restored instantly in the game store, or request lives from your friends.
Sometimes you can get free lives by watching an ad.