A literary device used to introduce background information about events, settings, characters etc. to the audience or readers. (Note: you cannot totally avoid exposition, but you should limit your use of it. It takes the reader away from the action of the story.)
Example: "Once upon a time, there were three bears. There was a Daddy Bear, who was very big, a Mama Bear, who was middle-sized, and a Baby Bear, who was very small. They all lived together in a little cottage in the middle of the woods. Their favorite breakfast was porridge. One morning, after they made their porridge, Daddy Bear said, 'Let's go for walk in the woods until it cools.' Mama Bear and Baby Bear liked the idea, so off they went. While they were away, a little girl named Goldilocks came walking through the forest and smelled the porridge..."