Stamina is a game mechanic which affects survivors by determing their movement speed based on the weight they are carrying and their fatigue. As a survivor moves, their stamina drops and consequently, the slower they move. Standing still or walking (shift by default) will allow the stamina bar to refill. Standing still allows the bar to recover faster.
At full stamina and with up to two bars of stamina drained, player speed is normal. After that a player's speed decreases by 5 units per second every bar.