So these two men are on a train ...

One man says, "What's that package up there in the baggage rack?"

The other answers, "A MacGuffin." 

The first one says, "What's a MacGuffin?" 

"It's an apparatus for trapping lions in Scotland." 

"But there are no lions in Scotland." 

"Well, then, that's no MacGuffin."

Alfred Hitchcock tells that story to illustrate the meaning of MacGuffin, who is credited with coining the term.

The word means "an otherwise meaningless object in a film or book that provides the motivation for the action; a flimsy excuse for an action."

The linguist Robert Beard explains: 

For an object to be a MacGuffin, it can have no significance itself; it cannot help us understand a character. Perhaps the most famous MacGuffin is the black statue in The Maltese Falcon. Humphrey Bogart, playing Sam Spade, wraps up the case by saying, "Oh, and I've got some exhibits: the boys' guns, one of Cairo's, a thousand dollar bill I was supposed to be bribed with—and this black statuette here that all the fuss was about." A more contemporary example is the briefcase in Tarantino's Pulp Fiction. The Ark of the Covenant in Raiders of the Lost Ark doesn't fit the definition since it has meaning outside the film and plays a key role in the film's climax.

You can use today's word in many contexts, Dr. Beard says: "I don't know why Macie had to go shopping. She's on a search for some skincare MacGuffin." In other words, Macie is ranging the mall with only the flimsiest of excuses for doing so. This word may also be spelled McGuffin: "The boss wouldn't let me leave when my project was finished, so I spent the afternoon rifling the filing cabinet for a McGuffin to get me out."