An example: Are you familiar with The Lord of the Rings? The story revolves around the One Ring, its significance, and how it's destroyed. But how did Frodo get the One Ring in the first place? ...that background story is in The Hobbit. You obviously can't write a full-scale novel in 15 minutes, but you could write the scene where Bilbo encounters Gollum and stumbles across the ring. That would be a background story that sets up the plot in Lord of the Rings.

Does that help?