The Shading Group is a collection of materials, textures, and lights that efficiently describe all the necessary attributes required to shade or render an image.
All of the nodes connected upstream (that is, farther to the left of the node) of the shading group contribute to the final look of surfaces in the scene. Adjusting any node’s attributes or connections causes a downstream (that is, farther to the right) reaction that can be seen in the final rendered image. It is not recommended that you manually try and edit the shading group connections. Instead, always use the higher level interfaces, like the Hypershade and shading groups editors, to ensure the connections do not become inconsistent which will result in the surface not appearing when rendered.
To learn more about these input and output connections, see Show node connections.