Freya,
It definitely doesn't mean why you are writing the book. However, it isn't something easily defined out of context.

In the calendar, Melissa gave the example of "The Hobbit" being a plot background story for "Lord of the Rings", because Bilbo's adventure led to Frodo having the ring in the first place.

Your story may not have a MacGuffin to write about, but somewhere before the beginning of your story, something happened that put your characters into the position of being the characters of your story. Perhaps it's the events that molded them into being the sort of people who would do what you have them doing. It might be the thing that made your antagonist become a villain. Or tragic circumstances that forced your hero to grow up in the care of distant relatives, causing an important character change. In Harry Potter, it's the events just prior to the beginning of the first book (details omitted just in case there's still someone out there for whom they would be spoilers) that put him under the care of his aunt and uncle.

...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown