It is supposed to be the plane of the floor with respect to the camera. The equation that you listed is the plane equation of the floor when the origin is at the camera center. At that point, x=y=z=0, so you are left with only D which should
represent the negative of the height of the camera off of the floor.

If you plug in other <x,y,z> coordinates, then you will get the distance from that point to the floor plane. This is the shortest distance to the plane, which means a vector perpendicular to the plane is made to your point, and then take
the length of that vector gives you the distance.