Above the Material Editor sample slots is the menu bar. Below and to the right of the sample slots are buttons and other controls that you use to manage and change maps and materials.
Material Editor tools below and to the right of the sample slots
Reflectance and Transmittance Display
Buttons below the sample slots (the "toolbar"):
Reset Map/Mtl to Default Settings
Show Shaded/Realistic Material in Viewport
Buttons to the right of the sample slots:
Make Preview, Play Preview, Save Preview
Controls below the toolbar:
Pick Material From Object (Eyedropper)
Name Field (Materials and Maps)
Type Button (Materials and Maps)
To use the Material Editor Options dialog:
To change the preview shape:
The new shape is displayed in the sample slot, with the material mapped to it.
The flyout gives you three options: sphere (the default), cylinder, or box. An additional custom object option is available if you define a custom object as described in Creating a Custom Sample Object.