Golf is difficult because we stand to the side of the ball during setup. This makes aiming difficult. Our eyes can fool us as to where we are lined up. We cannot look directly down the line. Another result is we are obliged to swing on an arc. Think about it. If we tried to swing the club straight up and down, the golf club would be vertical. This is steeper than the shaft angle at address. If we swing the club straight up and down the toe of the club would go into the ground.

If we swing too much around ourselves the club shaft is flatter than it was at address. We might top the ball or miss it all together. When talking about swinging on the correct plane we want to be between these two extremes. Ideally, this would bring the sole of the club level with the ground, or just brushing the grass at impact.