A MacGuffin is an object that is coveted by one or more characters. Important objects used in major works of fiction include the One Ring, Excalibur, Cinderella's Glass Slipper, The Holy Grail, Snow White's apple, etc. Important objects in a crime novel can be a major clue, such as a murder weapon or a body. They could also be a document or flash drive containing secret documents in a spy novel.
Objects and object-oriented goals as plot devices are frequently used to explain character motivations. Objects can be crucial to the plot even though the story or novel is really about the evolution of the characters. When one of these objects is missing and characters are desperately searching for it the object can be a MacGuffin. The hero may want to get this item before the villains. A classic MacGuffin object from myths is the Holy Grail. The Wikipedia entry says MacGuffins have sometimes been called plot coupons.
The MacGuffin term is attributed to Alfred Hitchcock who also popularized the term. TV Tropes has a big list of MacGuffin sub-tropes. According to the Alfred Hitchcock Wiki, Hitchcock described a MacGuffin as a "mechanical element" in a 1939 lecture at Columbia University.
Hitchcock said, "[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."
This video from ScreenPrism explores Hitchcock's MacGuffin concept and gives many examples from films, including The Maltese Falcon, Psycho, The 39 Steps, Indiana Jones and the Temple of Doom and Private Ryan. Psycho is a great example of a MacGuffin because Marion Crane decides to embezzle $40,000 from her employer and leaves town. The theft of the money is used as a MacGuffin. The $40,000 motivates her and gets her on the road out of town where she gets lost and ends up at the creepy Bates Motel.
The video also talks about how with some films the MacGuffin is too interesting for the audience to forget about it in act 3, such as The One Ring in The Lord of the Rings.
The ScreenPrism video talks about how George Lucas considers the Death Star plans to be a MacGuffin. These plans later ended up being reused in the brilliant Rogue One movie.
IMDB has a photograph list of movie MacGuffins. It includes Grays Sports Almanac in Back to the Future Part II, Unobtanium in Avatar and Milton's red stapler in Office Space.