When a character is injured, he or she temporarily loses hit points from his or her maximum total. A variety of ways exist to restore these lost hit points (such as magical healing or natural rest), not to exceed the character's maximum.
Zero Hit Points
When a character's hit points reaches zero the character is rendered unconscious and unable to act or follow player instructions. Should hit points become negative, the character is considered gravely injured and/or bleeding to death and once hit points reaches -10 the character is slain. Dying characters can normally be "stabilized" (their hit points returned to zero) by a fellow party adventurer, even during combat.
Character Hit Dice
First level characters begin play with a single hit die (dice roll) depending on their class and constitution bonus or penalty (with a minimum of one hit point). Each time a character gains a level, that character receives another hit die which is added (along with the character's constitution bonus or penalty) to his maximum hit points. Note that, after 9th level, characters receive a fixed number of hit points each level and no longer adds the constitution bonus or penalty.