A McGuffin (sometimes MacGuffin or maguffin) is a device that drives the plot, but has no real relevance.
A good example is the briefcase in Pulp Fiction: viewers can speculate on its contents, but the truth is it doesn’t matter what’s inside; it’s just something to drive the plot.
The term is often attributed to Alfred Hitchcock. In a 1939 lecture at Columbia, he explained:
It is the mechanical element that usually crops up in any story. In crook stories it is almost always the necklace and in spy stories it is most always the papers.