A blend-file can store previews, both for itself, and for some of its data-blocks. You can disable writing any previews when saving a blend-file using the Save Preview Images setting from the Save & Load section of the Preferences.
Blender saves by default a small preview of current scene in the blend-file. This will show in the Thumbnail view of the File Browser.
During its installation, Blender also adds a small tool to your OS, that will allow your system file browser to show those previews as file thumbnails as well.
Blender will generate automatically previews for some type of data, mainly shading-related ones (images, textures, materials, lights, worlds).
It can also store previews for scenes, collections and objects, but those need to be generated manually.
These previews can then be used by the Thumbnail view of the File Browser, when linking or appending data-blocks.
Refresh Data-Block Previews¶
Refresh all data-blocks' previews that can be automatically generated by Blender (shading-related ones), in the current blend-file. You still need to save the file if you want to write them on drive.
Clear Data-Block Previews¶
Clear all, a generic type of, or a specific data-block type of previews in the current blend-file. You still need to save the file if you want to clear them from drive.
Batch Generate Previews¶
Generate some data-block types' previews (you can choose which in its options), in one or more blend-file on your drive. You should not use that operator on the file currently opened in Blender.
This is currently the only way to generate and store in blend-files previews for scenes, collections and objects. Note that since this involves a lot of rendering, even of small sizes, the process may take some time to complete.
Batch Clear Previews¶
Clear some data-block types' previews (you can choose which in its options), in one or more blend-file on your drive. You should not use that operator on the file currently opened in Blender.