Chekhov's Gun is the act of placing an element into a story that seems small and unimportant at first but ends up a really important object. Similar to foreshadowing.

This device received its name from writer Anton Chekhov, who famously advised the writers of the world in 1911 that "If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there."