Cube World Wiki, cataloging the cubes.
Stamina is the energy that you use to perform actions like Dodging, climbing and using your hang glider. Your stamina is represented by a blue bar right above your HP and MP.
By putting skill points on your climbing and hang glider skills, you can make the stamina last longer.
Your pet also has a thirst meter similar to stamina. For more information on that go to the pet page.