But if I was to say that ........
A MacGuffin (sometimes McGuffin or Maguffin) is a plot device that motivates the characters and/or advances the story, but has little other relevance to the story.
The director and producer Alfred Hitchcock popularized both the term MacGuffin" and the technique. According to the Oxford English Dictionary, Hitchcock explained the term 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 most always the necklace and in spy stories it is most always the papers."
The element that distinguishes a MacGuffin from other types of plot devices is that it is not important what the object specifically is. Anything that serves as a motivation will do. The MacGuffin might even be ambiguous. Its importance is accepted by the story's characters, but it does not actually have any effect on the story. It can be generic or left open to interpretation.
The MacGuffin is common in films, especially thrillers. Commonly, though not always, it is the central focus of the film in the first act, and later declines in importance as the struggles and motivations of characters play out. Sometimes the MacGuffin is all but forgotten by the end of the film.
...... you might well assume that something I read recently has a doozy of a MacGuffin. I couldn't possibly comment.