Foreshadowing is planting clues early in a story to give readers some idea what’s coming up. They can’t be certain the clues mean something because everything brought up early isn’t foreshadowing a future event.
Foreshadowing can be as vaporous as mentioning knives a lot early in a story and then having someone killed by a knife at the end. You may only mention one particular knife and it becomes the one used in the murder.
Foreshadowing can also be heavy. Your novel could open with a serial killer at work on a seemingly random victim. As the novel progresses, the victims come closer and closer to the protagonist.
Foreshadowing doesn’t require that the reader gets any kind of clear idea that it was put there for them to see what’s coming.