While there are several planes in the golf swing, when someone uses the term "On Plane" they are typically talking about the club being on it's original shaft plane. The original shaft plane is the plane line that the golf club is sitting on at address. If you drew an imaginary line through the clubshaft and up into the sky you could see the plane that the club should trace throughout the swing. A swing can either be on plane, under plane or above plane throughout the backswing and downswing. Keeping the club on plane is the most efficient use of energy and is ideal for ultimate control and accuracy.