The SceneExplorer gives you a structural overview of the currently edited scene and is active among all plugins
Most plugins will deliver their own UI elements to modify the scene so the SceneExplorer is more of a global tool. The simple SceneComposer however heavily relies on its functions as other plugins might too in the future.
The SceneExplorer displays Nodes in a tree that represents the tree of Spatials in your scene. Spatial controls, lights and geometry meshes are also displayed in the tree.
Right-click a Spatial or Node in the SceneExplorer to add other Spatials like ParticleEmitters or Lights, you can also add UserData to a Spatial that can be read during runtime.