RootSceneNode - In Ogre, SceneNodes are created in a hierarchy manner. Every -SceneNode needs a parent -SceneNode to be shown inside of a scene. Not really every because then the hierarchy-tree would've been infinite. That's where the RootSceneNode word comes in. It's the most parent -SceneNode in each -SceneManager (every -SceneManager has exactly one -SceneNode which is the root) and it has no parent! It's like the god of all SceneNodes.
For an other point of view the -SceneManager is comparably to the parent of the root. Note that RootSceneNode is just a word, the RootSceneNode is actually a normal -SceneNode except it does not have a parent -SceneNode, hence the prefix 'Root'.
Note: You can create SceneNodes which has no parent, but in this case they aren't inside of scene until it they will be attached to an other SceneNode.