VRAM Usage
The material editor display the estimated VRAM (texture memory) usage of the selected materials.
You can select multiple ground shader materials and it'll count the total VRAM usage of those materials.
It will not count duplicates of the same texture across the different materials and texture slots.
Unused Textures
Unused textures are ones that the material has a link to, but the shader doesn't actually use. These textures will be uploaded with your avatar and will count to your avatars texture memory count.
When you take an existing material and switch the shader to a different one, Unity keeps links to the textures that the old shader used in case you want to switch back to the previous shader. This is a genuinely useful feature, except for when you have no plans to switch back to the old shader.
In this case, the texture memory usage indicator will show how many textures are unused but still linked to the material. It will also let you choose to unlink these textures, saving VRAM.