A "MacGuffin" is a plot device used to motivate the action and drama in a movie, but that is - by itself - completely irrelevant and unimportant. It's a gimmick. A classic example of a MacGuffin is the statuette in The Maltese Falcon. While the black bird is the center of the story, it really doesn't matter at all what it actually is. Is it worth money? Does it have magical powers? Does it contain a secret Communist recipe for borscht? Is it made out of chocolate? Who cares!

Although Hitchcock is famously credited with coining the term "MacGuffin" (and used them liberally in his films), the device has been around for as long as humans have told stories, all the way back to the Golden Fleece, which Jason and the Argonauts have to recover because the fleece, uh, has magical powers or something. I forget. But the quest itself makes for a rip-roaring good yarn! The objectives of quests in adventure movies, top secret documents in spy movies and the contents of saftey deposit boxes in heist movies are all MacGuffins.

In its purest form, however, a MacGuffin, is never fully revealed or identified, precisely because its actual identity is completely unimportant. So while a storyteller might use the mysterious nature of the MacGuffin to add suspense, the real answer to "What was in the wine bottles in Hitchcock's Notorious?" is "It doesn't matter." But it does matter if you want to get a good score on this quiz!

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References:
* In Search of the MacGuffin!
* Hitchcock and the MacGuffin