Health is a measure of physical resistance to injury. Every time you take damage from an attack or event it is subtracted from your health and must recover over time. Large amounts of damage to your health can cause major injury or death.
Health is derived from the average of Strength and Endurance plus 10 or $(STR+END)/2+10$ in full. This means a completely average person has a health of 10 or '10 HP'. HP can be increased above or below the basic value. Though the alternative is to increase the resistance to damage or your ability to take large amounts of damage without dying.