Feast your eyes on an video animating Hitchcock's already very animated discussion (heh) during a 1970s TV interview as to what precisely a MacGuffin is.
The infamous 'MacGuffin'! For those who aren't familiar with the word, it's an object/goal that sets the whole plot of a film into motion but has little bearing on the plot or deeper themes of a movie. Or to reference the far more concise, elegant definition of Hitchcock scholar Ken Mogg, it's 'just an excuse and a diversion' and is usually used in thrillers.
It was coined by legendary auteur Hitchcock's Scottish screenwriter friend, Angus McPhail, and in an anecdote Hitchcock told to illustrate the word, he compared the MacGuffin to a mythical 'apparatus for trapping lions in the Scottish Highlands' – which is to say, it could be 'anything – or nothing – at all' (Mogg).
The most famous MacGuffin of all time has to be 'Rosebud' in Citizen Kane – it powers the whole film and we have no idea who/what it is until the last scene. For a slightly more contemporary example (and one I find even more captivating than Rosebud), think of the suitcase in Pulp Fiction.
Anyhow, enough yapping – check out the video and give me examples of your favorite MacGuffins in the comments, whether Hitchcockian or not!