Lives represent the amount of games you'll be able to play. You will see the number of lives you have on the upper left corner of your screen, represented in the shape of a heart.
A life will be taken from your account each time you start a game and, if you win, you'll be able to keep it. As long as you win the game, you'll be able to keep your lives.
If you lose the game, you'll lose that life and you'll have to use another one to start a new game.
Bear in mind that you will only be able to use one life per game and the only way to use them is by playing new games.