Fluff are extraneous touches and details that can be used to reinforce the impression that the characters are in a dream. Fluff usually has no bearing on the denouement and most often manifests as things that have left a mark on the Dreamer's subconscious mind. Put more simply, fluff are those confusing little things that seem both natural and out of place during your dream but whose meaning sometimes becomes clear once you wake up and have time to think about it ("Oh! So, that’s where that came from.").
That is the definition of fluff as described in the Dreamwalker: Roleplaying in the Land of Dreams core rulebook. In retrospect we feel that it did not adequately emphasize the limitless possibilities and/or significance of fluff.