Stamina
Stamina is a limited supply of sprinting power. It is displayed as a bar on top of the HUD.
Stamina regenerates when not sprinting. Sprinting consumes stamina faster the more encumbered the player is. If encumbered 
, stamina will be consumed at a significantly higher rate. If overencumbered 
, walking will start to consume stamina at a rate proportional to the amount of weight above the 100% encumbrance limit. Moving on a road rather than offroad reduce stamina consumption rate ten fold, but it increases risk of getting run over.
When overencumbered the stamina bar turn red under 20% and the player will collapse for a moment until their stamina regenerates enough. It is better to stop sprinting and regenerate it, than collapsing.
Sprinting increases the player's speed by 33% when their stamina bar is full. The bonus is proportional to the amount of stamina left, it becomes null when out of stamina.
Vaulting an obstacle doesn't consume any stamina.
Swimming
When swimming, any movement consumes stamina. Standing still doesn't consume stamina but doesn't regenerate it either. If your stamina bar gets below ~12% you will drown. You can "sprint" while swimming. The more encumbered you are the slower you swim but a full stamina bar will always give you exactly 15.8s of swimming time, even if you sprint while overencumbered.
Therefore to maximize your total swimming distance you should always sprint while swimming and try to have as little encumbrance as possible. If you accidentally fall in water and fear you won't be able to swim to safety, your best chance is to drop all your gear and sprint-swim without unnecessary movements.
In vehicles
While driving Vehicles, the stamina bar will show the amount of Fuel remaining in the vehicle's tank.