Hit points, or HP, represent the amount of damage your character can sustain before having to return home to rest. For all characters the amount of hit points is determined by the Constitution bonus. The more constitution you have, the higher your maximum hitpoints. This can be temporarily boosted with Constitution potions, and certain adventure buffs.
Hitpoints can be regenerated 3 ways: with potions of health, by resting at home, or via friend buffs.
Potions: health potions can be used before an adventure, or during one to replenish lost hitpoints.
Home: whenever your character is NOT adventuring you will regain 1 HP every 2 minutes.
Friend Buffs: If your character is low on hitpoints your friends can click 'Heal' to give you a marginal amount of hitpoints, for free! Alternatively if your hitpoint bar is full you will gain a +1 (max of +2) for each friend that clicks the 'buff' option.