Hit Point recovery occurs organically with a full night's rest of 6 or more hours.
The amount recovered depends on the conditions of rest:
Unsafe Rest (cavern, dungeon, camp, etc.):
Recover 25% of your maximum HP after a full night.
Safe Rest (inn, stronghold, etc.):
Recover 25% of your maximum HP + your Constitution Ability Bonus after a full night.
Downtime (one full week without adventuring):
Recover all lost HP.