Thumbnail Texture is an automatically collected screenshot of the environment that is triggered when the map quality is very good, i.e., when there are lots of feature collected in a small area. The thumbnail is presented to the user when they want to load and relocalize their scene, and it is intended to help with relocalization.
You just put notes all over a large room and clicked save. When you go to load the scene and see your notes pop up, a thumbnail picture of a table in that room is showed to you. You point the device at the table and everything relocalizes instantly!