Back-story is stuff that happened way back when, which the author has suddenly decided you need to know, even though the story has already progressed to now.
You plan out your novel and compile all the information that the reader needs. Then you organize it. If you organize it chronologically, there'll be no back-story. But often authors begin a novel at a point they feel is more likely to hook the reader than the earliest point they want to bring in. Take a novel about Evil Editor destroying the lives of writers. The book begins with EE rejecting an author and the author attempting to murder EE but failing and goes on until the author commits suicide. Standard literary fiction. Now suppose the author feels it's necessary to show a scene from EE's childhood, in which his mother takes away his rubber duckie in the bathtub and gives him a Nazi U-boat. The author could make this scene a prologue and put it first, or it could be brought in later as back-story, like a movie flashback.
If your back-story is the most interesting part of your book, it shouldn't be back-story. On the other hand, if your back-story is boring, dump it or come up with a more interesting back-story scene to illustrate your point. Make one up. Often your back-story is something interesting that can be appreciated only after the reader knows your characters. If so, you don't want to jump into back-story too soon. But too late is also bad:
Detective: The murderer was Professor Plum in the conservatory with the candlestick.This author should have worked the tea spilling incident in early, when it seemed irrelevant, so the reader had time to forget all about it until being reminded by the brilliant detective, who never forgets any clue.
Cop: But . . . how did you know?
Detective: I read his diary in which, six years ago, he described how he had a hatred of tea ever since an incident in which his grandmother spilled scalding-hot tea on his hand. And since the victim was a tea merchant, it was an elementary deduction.
When possible, you might want to have some secondary character inform the main character of a past event. The event was in the past, but the character is learning it now, at the same time the reader is learning it, so it's not really back-story. It's when the narrator pauses the story to provide back-story that it can become an issue. Readers naturally want to know what happens next, and can grow irritated with repeated flashbacks. So if a large number of essential scenes are set earlier than the beginning of your book, maybe you should consider starting the book at an earlier time. But there are no rules; back story can be in sentence 1:
When Evil Editor was nine, his teacher, Miss Bilgerat, made him sit in the corner wearing a dunce cap just because he corrected her grammar. It was a humiliating punishment he would never forget, and at the age of forty-seven, with a lifetime of failure behind him, as he sat in his van outside Miss Bilgerat's home aiming a bazooka at her living room window, he wondered whether revenge would taste as sweet as he'd long imagined. And he realized: How could it not?As you can see, if your back-story is well-written and riveting, it's not going to bother us. We probably won't even notice that it's back-story.