Single Plane; the shaft and right arm are inline with each other. You address the ball in the position that you’d be in when you strike the ball. Moe Norman called it Natural Golf and Todd Graves coined the term Single Plane.
One Plane; the shaft and hands are on the same plane as the shoulders. This swing requires a shallowing move before impact because of the steepness of the shoulders in the backswing.
To find a much better definition I’d suggest YouTube. Todd Graves for SP and Jim Hardy for OP. Their very different concepts and I’m certainly no expert.