Flashforward (or prolepsis), is a literary device which reveals the events that will occur in the future. This means that it is an insertion of a later event into the chronological structure of a story. It is also a sudden jump forward in time; which involves the scene that interrupts the present action of plot to shift into the future. Flashforward is also a scene that takes the narrative forward in time from current point of the story in literature.
The difference between flash-forward and foreshadowing is that foreshadowing uses clues or hints of the possible outcome in the future, without any interruption. Flash-forward, on the other hand, is an interjected/inserted scene in a narrative, which takes the narrative forward in time. The events presented in a flash-forward are bound/likely/obliged to happen in the story. Foreshadowing predicts the future events, but those events do not necessarily take place in the future.