Once you have enough coins and several cards that are at maximum potential, you can evolve your monster. This can be done in the same way as summoning. You need to have is the proper amount of coins as well as the correct stones from the field area. You can return to the field and use all your field energy every single time you recover it. This can be done when you level up or when the remains idle to regenerate it on its own. You can build up a large store of stones to use whenever desired.