Waiter's tray refers to a racquet position during the serve where the face is parallel to the ground and the racquet is pointing backwards at the beginning of the forward swing. The racquet could act as a "waiter's tray" holding food and drinks. The racquet face sweeps up 90° to impact.
You usually get this position by using an E. fh grip on the serve. The serves can be quite fast, well into the 100 mph range, but they'll rarely go in at that speed because this type of serve produces no spin on the ball. The window between getting it over the net and hitting long is almost zero. The waiter's tray also telegraphs the direction of the serve so the receiver can start moving to where the serve is going before the ball is even struck (10s of milliseconds, but at +100 mph that much time counts).
Also, since this type of swing can't produce much spin, there person likely has no second serve to speak of, so they have to bunt it over.