Stamina is a limited supply of running power. It is displayed as a bar on top of the HUD.
When a Player is running their stamina will deplete. Standing still or walking will regenerate stamina. Stamina will deplete faster the more encumbered the player is. If the player is overly encumbered, stamina will be depleted at significantly higher rate. Running on roads depletes stamina slower than cross country running, but increases risk of getting run over.
When stamina is nearly depleted, the stamina bar will turn red and the player will collapse for a moment until their stamina regenerates enough. It is better to stop running and regenerate it, than collapsing.
Players can also tread water for 8 seconds (if not encumbered) until their stamina depletes.