ArcGIS items can have a thumbnail that appears in the item's metadata, and in Contents view in the Catalog window. A thumbnail is a graphic illustrating the data an item contains. Thumbnails draw quickly because they are snapshots; the data isn't displayed when you see a thumbnail.
Thumbnails can be particularly helpful when you are evaluating the items you've found with a search, especially when you are searching a geoportal or metadata catalog where you might not have access to the actual item and can't preview its data.
You must create and update thumbnails manually. You might update a layer’s thumbnail after changing its symbology. If metadata doesn't exist before creating a thumbnail, metadata will be created to contain the thumbnail; however, it will not contain any descriptive information aside from the thumbnail.