A player can start to bleed when he has taken damage from another player, wolf or bear. To stop bleeding the player can use a Bandage, Small Medkit or Large Medkit. As long as it is not Cold and the player is not Hungry, the player will stop bleeding eventually without any treatment.
There is a marker above your health on your hud that says "bleeding"
While bleeding the player begins to lose health. The blood loss will slow down and eventually stop on its own.