Descriptive stuff to set the ambience in a game
We don't seem to have a page for this, it's fan speak that has more or less graduated to official jargon. Does it warrant a page? Flavor Text sort of covers it but "fluff" is a little more expansive -- it's probably Omnipresent Trope territory, unless there's especially notable examples.
"Fluff" is a term for descriptive stuff in a game that helps set the tone. It is the opposite of "Crunch" -- actual rules and gameplay mechanics. Examples can include descriptions of a race or character's personality, the history of a nation or the lore behind a magic item. Although fluff doesn't directly impact gameplay, it often indirectly shapes it -- if your quest involves dwarves, it's probably going to end up in a mine at some point. The term originated in the late nineties when "theatrical" tabletop RPGs came into vogue. A number of systems were published with lots of descriptive stuff but very few actual rules, since they were intended to be "acted out" rather than played as such. This prompted accusations from the RPG fan community that these games were "fluffy," lacking substance. Although it started out as a derogatory term, nowadays it is neutral -- well written fluff is very important to assist Willing Suspension of Disbelief. Fluff and crunch have become industry standard jargon and not just in tabletop RPGs. Flavor Text is one common form that fluff takes in Collectible Card Games and MMORPGs.