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.