The planning scene represents the world around the robot as well as the robot state. This is maintained by the planning scene monitor shown in the next diagram, taken from the concepts section of its official documentation at http://moveit.ros.org/documentation/concepts/. It is a subpart of move_group, which listens to joint_states, the sensor information (usually point clouds), and the world geometry, which is provided by the user input on the planning_scene topic.

This is shown in the following figure: