Maguffin

Maguffin or McGuffin is a term for a motivating element in a story that is used to drive the plot. It actually serves no further purpose. It won't pop up again later, it won't explain the ending, it won't actually do anything except possibly distract you while you try to figure out its significance. In some cases, it won't even be shown. It is usually a mysterious package/artifact/superweapon that everyone in the story is chasing.

I'd first like to say that The One Ring in the Lord of the Rings trilogy is NOT A MAGUFFIN. Because we care about what happens to the ring, the WHOLE premises of the film is Frodo and Sam embarking on their quest to destroy it and EVERYTHING that happens to The One Ring and because of The One Ring is consequential and it all matters. If anything, The One Ring is more of a deus ex machina, used to get Frodo out of otherwise imminent death. Whereas, something like the $2,000,000 in No Country For Old Men is a Maguffin.

The term was popularized by Alfred Hitchcock, who actually credited one of his screenwriters, Angus McPhail, with the creation of this concept and the name for it, citing a particular school-boy joke:

A man is riding on a train when a second gentleman gets on and sits down across from him. The first man notices the second is holding an oddly shaped package.
"What is that?" the first man asks.
"A MacGuffin, a tool used to hunt lions in the Scottish highlands."
"But there are no lions in the Scottish highlands," says the first man.
"Well then," says the other, "That's no MacGuffin".