The input plane object is a plane in 3D space. It stores the orientation of a modeling plane (=view's input plane) and the grid origin. You can place input plane objects to those areas of the scene where modeling actions are happening frequently. You can then set the modeling orientation and grid origin quickly by drag and dropping a suitable input plane object from the select window into a view window. You may also apply the auto focus tool to zoom into the region defined by the input plane square.
For example, when modeling a room interior, it may be a good idea to put an input plane object on the ceiling, on the walls and on the floor.
The input plane property sheet contains controls for plane orientation (HPB angles) and the position of the grid origin included in the plane. The Current property is used whenever a view window is set to Lock to Input Plane state. When the current input plane is modified, the view updates automatically.