The MacGuffin is a plot device in which some object of the quest is merely there as an object of the quest, with no inherent value in itself, like the Maltese falcon in the film of the same name. The MacGuffin is always a red herring of some sort, because the literal object of the quest is never that important in the end. It could be anything at all. It is the narrative structure itself that invests it with meaning.
The word is associated with Hitchcock, but the principle is more universal than that. What makes the MacGuffin the MacGuffin is the transparency of its fakeness. "Here is this flimsy plot device for you to focus on; now, on with the show."