Lights On is a made up Youtube feature. Lights On are two lights facing crossing with each other, like in a stadium or stage. The two lights shine on the video screen and the video screen is blurry. This shows that the viewer of the video is liking it.
If you click on the "Lights On" icon below the video multiple times, the video screen will turn white and the screen will say "Error!" like the dislike feature, Eruption.