I it a plate, I don;t have one so I'm not sure how thin, maybe 1/8" or 3/16", that goes between the plenum and the manifold and distrubs the air more evenly to all cylinders.
A stock setup has more fuel going the the back two cylinders.
actually, the stock setup has more air going to 5 & 6 thus causing them to run leaner than the other cylinders.....in theory, the power plate corrects this and evens out the cylinders