Complex sentence: The "fundamental plane" is the geocentric normal plane of the shadow axis, in other words, the plane through the Earth's center perpendiculer to the shadow axis, that is the line through the centers of occulting and occulted body.
Split into simpler sentences: The "fundamental plane" is the geocentric, normal plane of the shadow axis. In other words, the plane through the Earth's center perpendicular to the shadow axis, which is the line through the centers of the occulting and the occulted body.