Shoulder plane is the angle of the shoulders as the golfer turns to the top of the backswing. In an ideal swing, the shoulders would turn 90 degrees relative to the tilt of the spine angle in the backswing (pictured incorrect above, correct below). A flat shoulder plane is when the shoulders turn on a more horizontal plane than the axis of the original spine angle at address.
A player must have several physical characteristics in order to eliminate a flat shoulder plane. They must have good range of motion in the shoulders and lat muscles. They need to have the proper amount of forward tilt at address. They need to have the ability to rotate the upper body around the lower body.