Lives represent the amount of Classic game mode matches you will be able to play. As soon as you start a new match, a life will be deducted from your balance.
It is important to keep in mind that you will only be able to use one life per game. This means that, if you lose, you won’t be able to use another life and will have to start a new game.
At the top of your home screen, you will find your current life balance.