You begin the game with five lives, shown in your heart-meter.
If you don't pass a level, you will lose a life. You will get the chance to buy extra moves, or try the level again using another life.
When you lose lives, one life is refilled for free every 30 minutes until your heart-meter is full again. If you don't like waiting, you can get a full set of lives in exchange for Gold Bars from the in-game shop.
Another way to get lives without using Gold Bars is to ask for help from your fellow Farm Heroes Super Saga playing friends. Creating a King profile, which connects you to the Kingdom of fellow players, allows you to receive and send lives from your fellow players. You can only send lives when you have some, but don't worry, sending lives doesn't affect how many you have!
If you have linked your game to Facebook you'll be able to ask for extra lives from those people you know on Facebook who also play the game.