Health Points (or HP) are a measure of a character’s very life force. Their decline could be due to all sorts of physical problems: injuries, disease, old scars, etc. Simply put, Health Points represent how much "life" the character possesses.
A creature's Size determines how many health points it has. Most characters begin the game with 30 Health Points, so long as they're about human-sized. Damage in combat lessens the character's remaining HP. When you rest, you gain back health. When health reaches zero, a character becomes unconscious. When health drops below zero, a character dies. It's curtains, ya hear? Curtains!
For more information about how to lose or gain health, see Chapter 4: Life and Death.