Once you install this add on, you'll see a new button in your browser shaped as a lantern. It will appear when you'll browse any streaming video website like Youtube or Dailymotion. When you click it, the background will get dark and you'll view better the video, like in theaters.
You can change settings like background color or darkness level.
If you usualy watch streaming videos, Turn Off the Lights is a curious add-on for Chrome that can be a good partner.