- the stuff that happened before your story starts
- all the vital information we need to orient us to the story
Backstory is sometimes also interspersed throughout the story. (Often, backstory needs to be interspersed throughout the story in order to maintain the emotion of the story - or perhaps the tension. But you need to gauge how much is too much and how much is just the right amount to accomplish your goal.)
Rule #1 regarding backstory = Don't give the reader too much!
We only need the information that clarifies or enhances the story in some way.
If backstory doesn't clarify or enhance the story in some way, we don't need it and the reader won't care about it.
If we don't need it and the reader doesn't care about it, take it out!!