Fluff text, also referred to as flavor text, is a short passage used to describe something in gaming. It has no bearing on the stats of the item or how it plays, but rather merely provides background information. Game systems such a Battletech and Dungeons and Dragons use it in sourcebooks to flesh out characters and history.
An example of fluff text (from the Introduction to BattleTech Quickstart booklet description of the COM-2D Commando): Primarily designed for reconnaissance work, Commandos are often dispatched singly or in pairs to scout ahead of main forces or monitor pursuing enemies. The Commando's two SRM systems provide it with impressive firepower for a scout 'Mech. This unusual configuration a Commando to direct heavy missile fire at a single target or fire missiles at two different targets simultaneously. Additionally, the dual launcher set-up prevents an enemy from knocking out a Commando's entire missile supply with a single lucky shot. Despite this firepower, the Commando's thin armor make it a poor match for heavier 'mechs, so most Commando pilots prefer to shoot and run when facing such opponents. An arm-mounted medium laser and articulated hands--which give the Commando an edge during physical combat against light and medium 'Mechs without hands--complete the design's weapon configuration.
Here the fluff text describes details such as usage and strategy with the 'Mech. However, neither of these things affect gameplay. In fact, with some items, fluff text is contradictory to the item's stats.