A learned skill that can't be passed on. The MC can gain it by choosing the Doctor profession at the start, and there are a few unique characters that have it. Otherwise you will have to hope that a wild character you can recruit will have it, there is currently no way of acquiring it for your characters in the game.
It gives the character the FirstAid ability on the Overworld (accessible by clicking their portait), which allows them to remove an injury from another character for 2 food and wood. It also gives them the Doctor activity while Camping, which helps resting characters restore more HP.