What do teachers mean when they say about a book It's the narrator's own experiences?

The term for relating one's own experiences is autobiographical (about himself).

The viewpoint of a narrator who is relating his own experiences (e.g. I went down to the village) is called first-person narration. If it is from the point of view of an observer (e.g. The men went down to the village), it is third-person narration.