Lives represent the amount of Classic mode random matches you will be able to play. As soon as you start a new match, a life will be deducted from your balance.
You'll only need lives to play with random opponents. If you want to play with your friends, no lives will be necessary.
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 time of starting a new Classic random game, you will find your current life balance.