Many board games and video games provide their own fictional setting, or are set in fictional settings defined by other media such as literature or film. Fluff, in the context of such gaming fiction, refers to information that provides setting and context for the game.
By definition, fluff is never part of the proper game rules. As such, fluff has no impact on gameplay and can technically be ignored by the players without breaking the game or its rules; it could arguably be described as 'soft rules'. As a result, the concept may not be applicable for role-playing games where the rules are at the discretion of the game master and implied, contextual or otherwise 'soft' rules may be deemed valid and binding.
In the context of fiction, fluff is essentially synonymous with 'Canon', i.e. the body of all fiction deemed to officially shape and define the fictional setting. It often takes the form of short stories or background information found in rulebooks and game supplements, but can take any form. In highly developed, popular settings which have evolved beyond gaming, fluff may include anything from gaming supplements such as sourcebooks to novels or even movies.
By contrast, in the same context the term Blurb is typically used for brief explanatory texts (for individual gaming pieces etc.). Where they do not provide gaming rules for their subject, blurbs also fall under the definition of the fluff.