HP stands for hit points. It reflects the state of the character's health — more precisely, how far away (s)he is from death. It is primarily affected by the Toughness attribute and character level, and secondarily by Strength and Willpower.
As in most computer RPGs in existence, characters die once they reach 0 HP. Otherwise, it does not tend to affect the character's performance in battle, though characters that get reduced to a few HP after an attack might end up stunned.
Another roundabout way to use HP in battle is by casting spells from HP.
Maximum HP is not completely determined by the attributes or level, but can be affected by other sources, such as blessed healing potions.
It is possible to heal hit points by doing the following:
- Waiting for the PC to heal naturally (speed is affected by race, presence and level of the Healing skill, as well as the Candle starsign)
- Drinking a potion of healing, extra healing or ultra healing
- Casting Cure Light/Serious/Critical Wounds or Heal
- Applying the First aid skill
- Equipping regenerating items