The coordinate plane is restricted to a plane parallel to the xy, yz or xz plane. If you want to slice with an arbitrary plane you need this variable type.
What this variable type does is to define a two dimensional coordinate system in a plane. To do that you need an origin O and two orthonormal vectors u and v. Any point in the plane can be written as O + a· u + b· v, and corresponds to the coordinate (a,b).
Instead of viewing the plane in terms of u and v, it is viewed in terms of the normal and the up vector. The up vector is the same as the v vector since that vector corresponds to the coordinate (0,1), but the normal is a more natural way to view a plane and the u vector can be computed by the normal and v by the cross product of v and n.
The variable monitor shows the origin and the normal and up vector as well as a graphical representation of the plane that you can rotate around to get a better sense of its location and relationship to the standard coordinate plane.