A creature can sustain a wound so grievous that blood will continue to pour from it after the initial injury. Left untreated, a bleeding creature will eventually die of blood loss.
Only certain types of attacks can deliver a bleeding wound. After receiving such a wound, a creature takes 1 damage per round. This damage comes directly out of the creature's HP, and no amount of Endurance, Guard, or Armor can help. The damage per round increases by 1 for every subsequent bleeding wound the creature receives.
A character with ranks in Healing can apply pressure to the wounds and stop the bleeding. See the Healing section in Chapter 6: Skills for more details. In addition, natural remedies exist, and characters can leverage any item which restores HP to immediately cease bleeding damage.