The Advanced Material Editor allows the user to apply materials from the Content Browser to the material that is currently being edited. To expand the content browser panel, click on the black arrow at the left side of the material editor.
More info on the Content Browser can be found here
Double click on any thumbnail to load the material in the editor.
It is possible to preview a material before loading it into the material editor. To do that, follow these steps:
Turning off the Interactive Preview button will revert the material to the previous one. While in Interactive Preview mode it is also possible to double click on a thumbnail and load it into the current material.