MacGuffin is a plot device that motivates the characters but whose specific nature is largely irrelevant to the story.
Alfred Hitchcock popularised the term 'MacGuffin' in conversations with François Truffaut, defining it as the object the characters are after, which the audience is told to care about chiefly because the characters do. Hitchcock's example was a gag about two men on a train; one asks what's in the package on the rack, and is told it's a MacGuffin, used for trapping lions in the Scottish Highlands. When the other says there are no lions in the Scottish Highlands, the answer comes back: 'Then that's no MacGuffin.' Famous MacGuffins include the microfilm in 'North by Northwest' (1959), the briefcase in 'Pulp Fiction' (1994), and the Death Star plans in 'Star Wars' (1977). The concept reminds us that plot is fuel, not destination — what audiences remember is character, atmosphere, and image, not the documents that started everyone running.