I think Nick Lowe (the Interzone film critic, not the rock star or the Marvel editor) was the first person to coin the term. It's a very useful way of thinking about a particular type of structure of a story.
It's the kind of story where the protagonist(s) is told to go and collect a bunch of objects. It's a very good way into a world, because it takes you all over, looking for things. Often, early on, someone will say something like, "A thousand years ago, Estragon The Dark Clown, for reasons that will never be adequately explained in this book or its many sequels, placed his power in The Funny Hat of Doom, The Big Red Nose of Darkness, the Wig of Desmond, and the Revolving Bow Tie of Light. It has been written, that only when these four objects come together will a Saviour arise to save Clowntown. You, Beppo, you must take this map (helpfully printed in the front of the book for easy reference) and nip around the book obtaining these four things (each the object of veneration by a different culture, each guarded by very different groups of people) at great cost to yourself and to the supporting cast, and then you must bring them back here."
"Me? But I'm not even a full clown. I'm just a popcorn boy."
"They say the gods smile on popcorn boys, lad. But quick -- the Vladimirian Army approaches across the hills. Winter will keep them penned down, but if you don't bring back those plot coupons by the Spring, it will mean death for all of us."
"And if I do? What then, wise old clown?"
"Then, Beppo, we will still need to find someone to wear the Hat, the Wig, the Nose and the Revolving Bow Tie. Someone who is truly a hero. But who that person will be, I do not know."
"Don't look at me like that. Remember I am but a humble popcorn boy."
"Hey kid -- you're leaving Clowntown a popcorn boy, but you're coming back a hero!"
There. Does that help? And have I ever actually recommended Diana Wynne Jones's TOUGH GUIDE TO FANTASYLAND for anyone who wants to avoid the cliches of Fantasy. (It's always stew, she points out, despite how long it takes to make stew and how easy it is to burn stew even with a gas range let alone a cooking fire, and never omelettes.)
(By the way, books I always used to suggest to people (actually to lend to people) who wanted to be writers, particularly SF writers, were Reginald Bretnor's The Craft of Science Fiction, Samuel R. Delany's The Jewel-Hinged Jaw, and I'd add to that a wonderful essay by Alan Moore from around 1985 (republished, I think, in the early 1990s in The Comics Journal.)