A web thumbnail will be the image that displays on the website when customers are shopping for your product. Web thumbnails can be an image of your cover page or different artwork in the form of a JPG file if available (a screenshot of your cover art from your computer can be sufficient!). The image below shows what a thumbnail will do for your product if you provide one:

The product on top is one that has provided a JPG in the "Web Thumbnail" field in the uploader while the second product in the screenshot did not.  If you do not provide one, a default image that appears on all products without a thumbnail will be provided for you (which is why it is an important asset even if it is optional).