The Inspector window shows additional information related to the node selected in the main editor area. It allows the language designer to offload some, typically less-important editor elements, from the main editor to reduce clutter and noise of the editor. Some MPS built-in languages, like the editor definition or the generator macros, rely on the Inspector window a lot.
The Inspector window always shows the name of the concept of the node selected in the main area and offers a link to open the concept declaration: The additional editor content in the Inspector window is provided by the language designer. From the user perspective it behaves the same way as the editor in the main editor area does.
To see the Inspector window for a node that is already shown in the Inspector, you have to manually invoke the Inspect node action from the context menu.