Sounds like something good to eat, but really it's a powerful plotting device. Alfred Hitchcock is famous for his usage of the MacGuffin, but it's certainly not reserved for mystery or suspense stories. Nor is it reserved for the screen.
Writing a story without a MacGuffin is doable; the story could even become something of a masterpiece, but using this plotting tool makes steering a lot easier. Many writers use the MacGuffin without noticing, but becoming aware of it and utilizing it to its full potential is the difference between taking a road trip with a map or driving aimlessly.
Hitchcock described a MacGuffin like this:
"The device, the gimmick, if you will, or the papers the spies are after..."
It's the thing--a document or painting, love or God, a treaty or a secret--that is of vital importance to the characters. The protagonist will have some interest in the MacGuffin, whether it be he/she is trying to obtain it or withhold it. Somebody is after something, and someone or something is in the way.
In Shakespeare's Hamlet, the MacGuffin in the tale told by the ghost of Hamlet's father. The ghost appears to Hamlet to tell him his uncle killed this father to marry his mother and become king. All else follows from the beseeching of this vengeful spirit.
In Shakespear's Henry V, the MacGuffin is the entire country of France.
In The Maltese Falcon, the MacGuffin is the black bird that Gutman, the villain, is after. Dashiel Hammett spends over 2,000 words explaining this foot-high falcon for a reason. If the bird isn't romanticized and made a big deal of, it wouldn't seem plausible to have Gutman use 17 years of his life hunting for it, or have half a dozen people die pursuing or protecting it. A MacGuffin has to be worth the drive.
In Hitchcock's view, the MacGuffin wasn't that important. It's merely a device to pull the characters through their journey. But it must mean the world to the characters, whose journey is of great importance to both writers and readers.
Some things to keep in mind when creating a MacGuffin:
The MacGuffin has to match the character. If your character isn't greedy, you probably don't want your MacGuffin to be gold. Maybe your character is competitive, though. A good MacGuffin could be something like winning a race or catching the guy before another girl does.
As mentioned earlier, the MacGuffin should seem plausible. It'll have to be something worth it, at least to the character. This takes both good characterization and good description of the MacGuffin.
