In the world of writing, "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 contributes to the revelation of character and theme."*
Do you use flashbacks in your writing?
* Janet Burrow quotes this from David Madden's A Primer of the Novel for Readers and Writers.