A spoiler is a summary or description of a narrative (or part of a narrative) that relates plot elements not revealed early in the narrative itself. Moreover, because enjoyment of the plot depends on dramatic tension and suspense, this early revelation of plot elements "spoils" the enjoyment that consumers of the narrative would otherwise have experienced.
See also: Wikipedia contains spoilers
A spoiler, also called an air foil, is a device sometimes found on the back of cars that improves traction. The spoiler is basically like the wing of an airplane, but it's upside down, so the air pushes it downward instead of upward.