Fiction writers at writing workshops often hear, "Get rid of the exposition." "There's too much exposition in here." "Exposition" --it can be good, but in fiction class it was always bad, and I understood the concept from examples, but never had it explained to me in a convenient nutshell.
As often happens, the word provides its own understanding. Think of the word as "Ex-position." "Ex" means "out." So the word means "out of position." Exposition is a capsule of description or dialogue that doesn't really fit in its time and place, or fit the character. Example:
"Amanda thinks she's Jane Austen, the famous English writer born in 1775 who wrote Sense and Sensibility and then Pride and Prejudice, and then Northanger Abbey which spoofs the Gothic novels popular at that time. Look, there's Amanda now."
That's fine information in that first sentence, and all true, but it doesn't belong there. It's out of place.
Exposition doesn't have to be factual. It can be fictional. It's an authorial intrusion: background information that has been foregrounded in a place in the narrative where it doesn't fit. The author is unsure about a choice he made -- maybe, he thinks, he'd better explain who Jane Austen was so we will all get it! -- so he patches up the narrative with further information, hoping for the best.