Fluff can mean several related things:

When you fluff a pillow, you shake it and pat it to make it soft and puffy again. After sitting on a couch cushion all afternoon, you might fluff it up so the next person gets a comfortable seat. The word captures that light, airy quality: bird feathers are fluffy, as are cotton balls, dandelion seeds, and freshly fallen snow.

The word also describes writing or speech that sounds nice but lacks real substance. If your book report is supposed to be two pages but you only have one page of actual ideas, you might be tempted to add fluff: extra words, fancy-sounding phrases, and repetitive sentences that take up space without saying anything meaningful. A teacher can usually spot fluff immediately because it feels empty, like biting into a pastry that's all air and no filling.

You might hear someone say they need to fluff up their essay, meaning they need to expand it. But there's a difference between adding genuine examples and explanations and just padding it with fluff. Real substance has weight and meaning. Fluff just takes up room.