There are a number of common plot devices that use objects to advance a story in a convenient way, by providing motivation, conflict or a needed resource later on. These plot tokens can be used to good effect, but all too often they become a lazy stop-gap to save an author from actually thinking out a stronger narrative structure.
The best known is Alfred Hitchcock’s ‘MacGuffin’. This is a common staple of thrillers and crime stories, but it crops up in fantasy as well. The MacGuffin is an object of no inherent importance to the plot, but which all the major characters desire. It gives a reason for the protagonist and antagonist to come into conflict with one another, but usually ends up forming no more than an afterthought to the end of the story. Many MacGuffins are never actually described. “We must find the Last Seal before the Dark Lord gets to it.”
The ‘plot coupon’, named by Nick Lowe, is an object which is the only item that will allow the protagonist to achieve victory, with that victory coming chiefly through possessing or using the item. Plot coupons are often broken down into several pieces; the joke then states that when the hero has collected enough of them, she can trade them in for a victory. “But now I have the Amulet of Rodney! Prepare to die!”
All the various forms of Plot Token have their place, but be very cautious. They are often a sign that the story really has not been properly thought through, or that the plot is lazy and malformed.