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.
EDP445: I scared a 7 year old girl
EDP445: sends fucked up messages to an underage girl
people online: this is definitely one of the best examples of foreshadowing
guy: well guys i'm going to be heading home, just had a close call with the foreshadower and i'd like to avoid any mishaps.
Pal: o well good luck, man.