Here’s good ol’ Wikipedia: In fiction, a MacGuffin (sometimes McGuffin or maguffin) is a plot device in the form of some goal, desired object, or other motivator that the protagonist (and sometimes the antagonist) is willing to do and sacrifice almost anything to pursue, protect or control, often with little or no narrative explanation as to why it is considered so important. The specific nature of a MacGuffin is typically unimportant to the overall plot. [...] The MacGuffin technique is common in films, especially thrillers. Usually the MacGuffin is the central focus of the film in the first act, and then declines in importance as the struggles and motivations of characters play out. It may come back into play at the climax of the story, but sometimes the MacGuffin is actually forgotten by the end of the story. [...] The specific term “MacGuffin” appears to have originated in 20th-century filmmaking, and was popularized by Alfred Hitchcock in the 1930s, but the concept pre-dates the term in film as well as in literature. [...] Hitchcock popularized both the term “MacGuffin” and the technique, with his 1935 film «The 39 Steps», an early example of the concept. Hitchcock explained the term “MacGuffin” in a 1939 lecture at Columbia University: “[We] have a name in the studio, and we call it the ‘MacGuffin’. 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”. Interviewed in 1966 by François Truffaut, Alfred Hitchcock illustrated the term “MacGuffin” with this story:
It might be a Scottish name, taken from a story about two men in a train. One man says “What’s that package up there in the baggage rack?”, and the other answers, “Oh, that’s a McGuffin”. The first one asks “What’s a McGuffin?” “Well”, the other man says, “It’s an apparatus for trapping lions in the Scottish Highlands”. The first man says, “But there are no lions in the Scottish Highlands”, and the other one answers, “Well, then that’s no McGuffin!” So you see, a McGuffin is nothing at all.
Hitchcock related this anecdote in a television interview for Richard Schickel’s documentary «The Men Who Made the Movies» and for Dick Cavett’s interview. According to author Ken Mogg, screenwriter Angus MacPhail, a friend of Hitchcock, may have originally coined the term.