In layman's term, flash-forward is when events sequences in a story are interrupted by the insertion of an event that will happen in the future.
I think one way it affects the plot is that the reader or viewer are seeing an event that will happen in the future. Typically authors don't allow the readers to do that because they want to add suspense but I think having flash-forward in a way can create suspense as well. Readers get a glimpses of what's going to happen but they aren't sure exactly when those events will come to play.
Hopefully this helped and good luck. :)
