A flash back is just that, a flash backwards in time, giving the reader a glimpse at an earlier event. It is a form of exposition in that it should convey necessary information. For example if your mc has been treated badly in the past and this has influenced who he/she is as the story begins then a flashback could be very helpful in showing or telling the reader what the mc endured, thereby contributing to character. But don't confuse a flashback (a glimpse of the past) with backstory ( everything that happened before the story began). Flashbacks should be brief, and they will be most effective "if the very fact of their occurrence contibutes to the revelation of character and theme."*
This is a flashback from my current work in progress, namesake:
"Arlen had known they would ask her to speak. But every time she tried to think all she saw was the barn going up in flames. Even now she could almost smell the way the air had been fragranced with the last scents of summer and smoke from the burning hay. She could see in her mind’s eye the way the fire had grown so quickly, leaping like an acrobat, orange flames climbing like vines through the structure. And she remembered the heat of it – so hot – like hell, she thought, wondering, just for an instant, what it might be like to walk into that fire and burn. But what could she say? How could she make them understand how seductive the flames had been? How could she explain the way it felt to be so close to burning?"
* Janet Burrow quotes this from David Madden's A Primer of the Novel for Readers and Writers.