Wikipedia definition of the Alfred Hitchcock concept of a MacGuffin.
In fiction, a MacGuffin (sometimes McGuffin) is a plot device in the form of some goal, desired object, or other motivator that the protagonist pursues, often with little or no narrative explanation. The MacGuffin’s importance to the plot is not the object itself, but rather its effect on the characters and their motivations.
The MacGuffin technique is common in films, especially thrillers. Usually, the MacGuffin is revealed in the first act, and thereafter declines in importance. It can reappear at the climax of the story but may actually be forgotten by the end of the story. Multiple MacGuffins are sometimes derisively identified as plot coupons.
Definitive List of Hitchcock McGuffins
Also, Hitchcock Mania has a collection of stills from all of the Hitchcock films, including several MacGuffins.
Rope MacGuffin – The wrong hat
Dial M for Murder MacGuffin – the spare key to the apartment
Strangers on a Train MacGuffin – The lighter implicating Farley Granger