click right mouse button on the preview icon of the material
Click on Inspect from the appearing dropdwon menu
If you select a material, in the project browser or clicking directly on an object into the sceneviewport, before opening the Material Editor, the editor will be opened with the selected material data loaded.
Once you have opened the Material Editor, 2 panels are opened:
When material editor is opened, if you click on an object surafe into the scene viewport, the related material data is loaded into the editor. The same happens if you select a material from project browser.
For a complete description of all the tools of the Material Panel see the Material Panel paragraph.