Stamina is a hidden attribute that is affected by Will and the Distance Runner feats. Stamina effectively limits the amount of times you can use skills, special actions and stairs. You will recover stamina every couple of moves, but actively 'R'esting is more practical.
When your character uses too much stamina, they are inflicted with the Tired status effect (and later Very Tired, and VERY Tired), which reduces speed and causes a chance for further attempts at using the ability to fail ("You're too exhausted!"). Being VERY Tired will impede your attempt to fight or move altogether.
If you are tired and attempt to use stairs, you may "lose your step and roll down", taking damage.