Exteriority is a characteristic of being on the outside of something. In books, a character's exteriority is what they do in the outside world, rather than their thoughts and feelings.
Exteriority is a fancy word for "a state of being external or outside." When an architect talks about a building's exteriority, they are literally referring to its outside elements — exterior walls, decks, porches, and roofs — in contrast to the interior rooms. In literature and philosophy, this noun is used for things that happen to a person in the physical world, not their reactions, sensations, or emotions. The Latin root means "on the outside."