A windage tray is a louvered or contoured sheet metal device that fits between the rotating components of the engine (crankshaft and conrods) and the oil in the sump. It acts as a barrier to prevent oil being whipped up by the spinning crank mass, thereby creating heavily aerated oil in the engine. It has a twofold advantage; the parasitic losses created by the whipped up oil cloud are quite high, costing horsepower; and the second advantage is that the oil is kept in a more liquid state in the sump rather than a frothy, aerated mix so therefore your oil pump can supply the engine with a plentiful supply of liquid oil rather than a gaseous oil/air mix - your bearings will love you for it!