Fluff is a soft, light puffy or feathery material. Your pet angora rabbit is covered in fluff, and when you blow the down off a dandelion, you can call that fluff too.
A department store window designer might use white cotton fluff as snow in his holiday display, and a bored dog might pull the fluff out of every pillow in the house while you're at work. The word fluff is also a verb meaning "to shake or ruffle until something looks fuller." It's also a great way to describe something meaningless or frivolous: "I had high hopes for the newspaper article, but it was just fluff."