This helps locate the point you wish to skip to, should you wish to find a particular section.
If the video has only been recently completed encoding, these images may not have been generated yet and so you will only see the time-code. These preview images are an additional feature, so they are generated in the background later on. You do not need to do anything to enable them - they are available for all videos within the Vidbeo hosting platform right now.
Updated: January 14, 2015