Thumbnail sketches are often sketches done by an artist. They are quick and fast with no fixes or changes involved in it. A thumbnail sketch is used to get the basic concept or idea of the sketch. For example, if the artist feels like creating a drawing, the artist very well might create a few thumbnail sketches to get more of an idea that they want to draw. They also may do a thumbnail sketch to see how they want objects in their piece to line up.
this gave me 100% on ed2020, so I hope it is useful to others.
