Foreshadowing is a dramatic device in which an important plot point is mentioned earlier in the story to return later in a sinificant way. This is said by Hermione Granger in A Very Potter Musical.
Snape: A person can however be a horcrux.
Harry: What's a horcrux?
Snape: I'm no even going to tell you Harry, you'll find out soon enough.
You find out later, Snape knew there were horcruxes concerning Harry. This exemplifies foreshadowing.