The stamina potion is an item held in the 2 left slots of the player's inventory. This potion is used to refill your Stamina or add a whole other line of stamina. The potion is white, and contained inside of a bottle. The original stamina potion/idea designed by Tytonawy Janusz (who is one of the builders in the game) was actually blue but it was later changed by Rezoner.
Stamina potions are mostly useful for running, long jumping, and getting to locations while preserving more stamina.