Flashback is a literary device wherein the author depicts the occurrence of specific events to the reader, which have taken place before the present time the narration is following, or events that have happened before the events that are currently unfolding in the story.
Flashback devices that are commonly used are past narratives by characters, depictions and references of dreams and memories and a sub device known as authorial sovereignty wherein the author directly chooses to refer to a past occurrence by bringing it up in a straightforward manner. Flashback is used to create a background to the present situation, place or person.
Back in the day when Sarah was a young girl…
You can see flashbacks used very often in movies. For example, it is common in movies for there to be a flashback that gives the viewer a look into the characters life when they were younger, or when they have done something previously. This is done to help the viewer better understand the present situation.