Each player’s swing shape is unique, but there is only one important ingredient to your specific swing shape: the plane of your swing.
The plane of your swing can be thought of as somewhere in between a merry-go-round and a ferris wheel. Each plane has a horizontal and vertical element to it as well as the plane direction, ie. inside-out or outside-in or at the target.
When you’re said to be “off plane,” it means your swing either became too much like a ferris wheel or too much like a merry-go-round.