HP is a measure of how much damage an object can take before it is razed or killed. If the HP reach 0, the unit (building) dies (collapses) instantly. Various kinds of armor protect from some damage and therefore prolong the life of a unit or building. Also, there are a lot of technologies that can improve the HP of various units and structures.
The Explorer (in Age of Empires III) and campaign heroes in Age of Mythology faint when their hit points reach 1. They are therefore immortal and can be revived after a while. Animals also remain as corpses and can be used to gain food until they run out or a natural decay eliminates the food available.
Regaining hit points Edit
Some units generally referred to as healers have the ability to heal friendly units up to their full health. That usually goes rather slow, but the more healers work together, the faster the healing goes.
In Age of Mythology, units cannot be healed by more than one healer, while some special and uniquely available units like the Son of Osiris, Nidhogg, or Titan cannot be healed at all. Meanwhile, the god power Regeneration, the Healing Spring-building created by a god power, and the Chinese myth unit Qilin are capable of healing all units within their range at once.
In Age of Empires II, units garrisoned inside a building also automatically regenerate their hit points slowly. Units garrisoned inside Castle regenerate hit points twice as quickly than in other buildings. Researching Herbal Medicine quadruples the rate at which units regenerate inside buildings.
Buildings, siege weapons, and ships cannot be healed by a healer, but can still regain their HP by being repaired. As opposed to healing, however, repairing costs resources, but only half the initial price. If a building is critically damaged, all garrisoned objects and units are ejected. They may reenter that building once it has been repaired.
In Age of Mythology, siege weapons and ships cannot be repaired but healed.
In Age of Empires III, siege weapons can be healed, ships are healed at the Dock and buildings can be repaired by paying a price.