Foreshadowing is kinda like alluding to something that is going to happen later in the story without completely giving it away. One of my teachers quoted someone by saying, "In a play, if there is a shotgun on the wall in act one, someone will have fired that shotgun by act three." Everything is the story is there for a reason. If it seems like a detail that is odd or out of place, it will probably play a part later in the story. It can also give hints to the reader about what is coming up so they have to opportunity to "figure it out", or at least speculate about what is coming up in the story, so that they will want to find out what happens. Hope this helps :)