When the video doesn't have 16:9 aspect ratio then HTML5 player (used by youtube) will add black bars to the video.
This is often the case with older TV series / movies which have 4:3 aspect ratio.

This style simply stretches the video to fill the entire player area.
You can have it enabled or disabled as needed.

If you like this style you may also like
http://freestyler.ws/style/145049/youtube-real-theater-mode