Stamina is used whenever you partake a mission, quest, or event. Basically, this gauge limits how many times you can play. STA is restored automatically at a rate of 1 point every 5 minutes. The maximum STA is increased whenever the player Ranks up. Ranking Up also completely restores the STA gauge. You can also spend a Dragon Stone to instantly recover your stamina though I will personally suggest against this, knowing that Dragon Stones are very, very valuable.