Stamina is used as you play through the game. It will refill on its own as time passes, but you can also refill your stamina instantly using Arelium (as in the image) or supply tokens you've collected during your adventure or have purchased.
You can see the stamina you have left in the upper right of your screen when you're in the arena or when viewing the world adventure map. Tapping on the + next to your stamina will allow you to refill your total stamina.
There are two types of stamina.
: Used for each battle in the arena. Each battle costs 10 stamina.
: Used to enter a portion of an adventure. You'll see the cost before you begin each map section: