HP, short for Health Points or Hit Points, is one of the three basic forms of energy a character has, and considered by many the most important. If a character reaches 0 HP, they will die. The purpose of attacks is to lower HP, and the purpose of Food and healing spells is to replenish it.
Players have a maximum value for their HP. This value is proportional to their STA. Some classes have a higher HP to STA ratio than others. The maximum HP a player has can also be raised by buffs and awakened or statted equipment.