(or Girl) is a colloquialism from television fandom
and fan fiction
which identifies the character who seems to be there mostly to allow the writer to explain things
which there is no other easy way to explain. These can be plot points, or the inner thoughts of the other characters, which are revealed to the audience when they tell them to this character.
In first-person detective shows, the lead is usually Exposition Boy, and the concept is less necessary (and less clear). It is common in James Bond movies for the villain to voluntarily step into the role of Exposition Boy. In the Austin Powers films, which are at least in part parodies of the James Bond movies, the role is taken by the character of Basil Exposition.