Hitpoints, also called health points, or abbreviated hp, is the amount of damage you can take before you die. When your character's hitpoints reach zero, your character will lose all of your items in your inventory and respawn back at the entrance of Buskingham.
Your hitpoints is the equivalent of your Combat level. You can raise your hitpoints each time by one every time you level up in combat. The more hitpoints there are on a character, the more damage the character can take before he/she dies in combat.