What is a RED line below the video thumbnail indicating?

Hi everyone! Im noticing these red lines at the bottom of my videos thumbnail. http://imgur.com/gallery/LIIw4. I cannot seem to find an answer to this. The videos plays fine and there are further no other issues. What is this indicating?

From a quick Google search:

"they're your progress on the videos. half red line means you've seen half of the video, full means you finished it"