Both are similar to each other. However, foreshadowing hints at the possible outcomes in the future without any interruption in the story and using events or dialogues of characters in the present time. Flash-forward, on the other hand, is an interjected scene that takes the story forward in time. Flash-forwards are bound to happen in the story because it is showing you the future of the story, but foreshadowing is just hinting and predicting future events, so they may or may not happen.
hope this helps!
