The planning scene (planning_scene::PlanningScene) is the central class for motion planning in MoveIt!. A planning scene represents all the information needed to compute motion plans: The robot's current state, its representation (geometric, kinematic, dynamic) and the world representation. Using this information, things like forward kinematics, inverse kinematics, evaluation of constraints, collision checking, are all possible.
The planning_scene::PlanningScene class is tightly connected to the planning_scene_monitor::PlannningSceneMonitor class, which maintains a planning scene using information from the ROS Parameter Server and subscription to topics.