Many abilities in the game can grant a character the ability to heal through various means. Healing is primarily done by a Medicus, via potions that an Alchemist can craft, and/or some rituals such as the Circle of Healing ritual.
Healing done by a Medicus can take several forms. They have the base ability to call Heal, which restores +2 HP (up to your base maximum HP) as well as apply a Poultice that grants +2 HP once they reach their second level ability Apothecary. Only a Medicus with this ability can apply a Poultice. Please see the subpage for Poultices for more information.
Potions, on the other hand, can only be created by an Alchemist but can be consumed or administered by anyone.
Several abilities in game will heal you (add points of HP back to your pool), but none of those abilities will interact with Temporary HP. Once Temporary HP is lost, it cannot be restored by healing. Sources that increase your HP should include whether they are adding to your Base HP or are Temporary HP. Common sources of Temporary HP include potions, the Ranger's Hunter Mark ability, the Druid's Animal Aspect ability, HP gained from the Bard's Rousing Performance ability, and some rituals. As a general rule, things that increase your Base HP include armor, enchanted items, the Champion's Tough ability, and other abilities that note that your base HP is increased.