Thumbnails are previews of deviations. Thumbnails are displayed in any gallery-related page in deviantART.
Thumbnail codes can be found on every Deviations. They're a way to share art quickly and easily using a preview throughout deviantART. Thumbcodes can be found on the right hand side of a Deviation, and is found underneath the "Group" display on the Deviation (i.e. :thumb299438170:).
For a bigger display of thumbnails use :bigthumb299438170:.
Big thumbnails (Beta feature)Edit
Recently the staff announced that Beta testers can now test a new feature of bigger thumbnails. DeviantArt has many representations for how unoticiable is pixel art and emotes. On the other hand,longer and wider deviations can now be zoomed in without having to downloading the full image.