There are many planes in the golf swing such as the shaft plane, the shoulder plane and the rear elbow plane. In general when people talk about swing plane they are referring to the shaft plane. The shaft plane is a 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. The shaft plane is determined by a combination of the design of the golf club and the player's posture.