Hit points are a measure of a player's physical endurance. When you are hurt, you lose hit points. Poisons, fires, and bleeds cause you to lose health over time, while other forms of damage directly remove your health.
You may gain your hit points back (up to your personal maximum) by either naturally healing over time or having active healing skills (such as 'first aid' or the Jedi skill 'heal') performed on you.
Players without any healing skills may find items such as Transdermal Patches and Bandages useful for healing, or can pay doctor NPCs to perform surgery on them. Healing skills may require items in order to use, and can be limited by your body's capacity to accept bacta. You may increase your overall hit points by gaining levels in any of your guilds.
Some skills, like the Gamorrean racial ability 'frenzy', give you a temporary boost to your total hit points. Others, like the dark Jedi ability 'anger', temporarily replenish your health, but the health is taken away again after their timers expire.