A flashback is a transition in a story to an earlier time, that interrupts the normal chronological order of events. A flashback in a movie might show what happened when a character was younger.
Flashbacks are often used for comedic effect, to prove or contradict something in the present. For example, if a character in a sitcom claims he does the dishes every single night, flashbacks could show him ignoring a giant pile of dirty dishes on various occasions. Flashback can also mean a sudden, vivid remembrance of past events, like having a flashback to your kindergarten days if you go visit that classroom today.
