How we change what others think, feel, believe and do
Presentation of the 'story so far' or other aspects of related elements that have some relevance to this story but which will not be covered in detail.
A common way of doing this is to embed it in a conversation between two or people, where one person explains what has been going on.
A play starts with an actor alone on stage talking to himself about what has happened, perhaps bemoaning his fate.
Movies start with a scrolling storyline to tell what has happened so far (eg. Star Wars).
A character is introduced and two other people gossip about her.
A villain explains to the captured hero how the crime was committed before the hero is about to be executed.
The exposition is useful device that allows the author to quickly give information to explain extra detail without having to cover it in normal story format.
When a play starts with an actor doing an exposition, it is known as a Prologue.
When this is not done well, it is sometimes known as a Plot Dump or Info Dump. This typically happens when a character mentions another person and then adds detail that would not be given in normal conversation.
When this is done in detail it is also sometimes called an Idiot Lecture, as it is done in a way as if the reader is an idiot.