Taken from a 1972 interview with Dick Cavett, some folks have animated Hitchock's explanation of what exactly a "MacGuffin" is; basically an empty parcel that moves a film along. As he told Francois Truffaut in 1962, "North By Northwest" is perhaps his purest example of this: "The picture is about espionage, and the only question that’s raised in the story is to find out what the spies are after. Well, during the scene at the Chicago airport, the Central Intelligence man explains the whole situation to Cary Grant, and Grant, referring to the James Mason character, asks, 'What does he do?' The counterintelligence man replies, 'Let’s just say that he’s an importer and exporter.' 'But what does he sell?' 'Oh, just government secrets!' is the answer. Here, you see, the MacGuffin has been boiled down to its purest expression: nothing at all!"
Anyway, give it a look below followed by some nice excerpts of the 1972 interview with Cavett. [OpenCulture]