First of all, what are comic thumbnails? No, it's not the thumbnail of your comic cover that is displayed in tapas listings.
Thumbnails are very small drawings, that allow an artist to quickly illustrate the contents of a comic page with very little effort. The drawings are very low quality, so, mostly wireframe/blocky people and simple shapes.
I did a bunch of them this week for my latest chapter, it didnt only allowed me to have an idea of the pages layout and content, it also provided me the chapters page count, which is important if you're planning a comic book, you can do the thumbnails until the book is filled even before working hard on the pages.
Here's an example of a page thumb, I usually draw several of these on the same sheet of paper to form a chapter:
