The plenum is the metal piece on the top of the intake which the throttle body bolts to. It directs the air from the throttle body down into the plastic intake. The aftermarket plenums I believe feature larger internal (polished and/or ported) diameters for better flow. I think gains are minimal. It is just one of those mods that can add a little bit more to a car. I would save money and dump it into a full new intake before getting the plenum, myself.