Video Preview Thumbnails (also known sometimes as "media index files" or "BIF" index files) are basically a series of low-resolution images from a media item. These images are used by some Plex apps when you move back and forward in the app with the timeline scrubber. There, they display a small image corresponding to that time in the video to let you better find the spot in which you're interested.
The images from the media index are also displayed in the "Now Playing" screen of the Plex Web App when a user is streaming content. This allows you to visually see where the user is in the video beyond simply seeing the progress bar.