Flashbacks can play pivotal roles in any story, whether it be fiction, nonfiction or a script. So what is flashback in a story? Some folks confuse it with backstory, but the time constraints of a flashback don't allow us to share too many details. They just allow us to reveal tidbits.
Here's a great explanation of writing flashbacks and backstory offered up by Ron Rozelle in Write Great Fiction: Description & Setting:
A flashback is a sudden, brief relocation to a previous time and then, just as suddenly, a return to the present story. Flashbacks can hint at backstories, but they aren’t backstories themselves. A backstory is a longer trip (in fact, sometimes backstories make up most of a story or even a novel).
In a flashback, a character is usually reminded of something or someone from his past. The smell of cabbage cooking might cause him to see a kitchen that he hasn’t actually seen in years and years. Or you might have a character who looks over at his wife of 50 years and, in just the right light of a nice afternoon, sees her as the teenager he married.
Flashbacks come in handy when you need to infuse a clue or two into a mystery or when some character trait needs to be enhanced or explained. Let’s say you have a fellow in your story who doesn’t like dogs. Your reader wants to know why, so you lead her along for a while and then give her a nice little flashback, in which the man recalls being bitten by a dog as a child.
Flashbacks are quick. Backstories, because they drag in the baggage of a character or a situation, are longer.
Backstory, when layered effectively, can be a good way to establish setting and provide description. Diverting your readers’ attention away from the here and now allows you to focus on times and places that give deeper insight into a character or a situation.
There you have it. And now that you know, you can focus on and improve these intricate parts of your writing. Your stories and manuscripts will be that much better because of it.