Your background story ideally happens pre-novel. These are scenes or stories that take your character into a setup for the novel that you will write.

For Harry Potter, an antagonist background scene might include a strategy for Snape to infiltrate Hogwarts. This might include a conversation wtih Voldemort, a way to get his references together (or however someone applies for a magical job, and to get that important interview with Dumbledore.

For the protagonist background, something that shaped Harry toward this moment when the novel would begin - but remember that the novel starts the night of the accident when Harry's parents died. So it might be about his parents preparing to bring a child into the world while worrying about this great threat Voldemort poses and the idea that they would have to go into hiding.

Does that start to make some sense?