When one watches one video on YouTube then the viewer sees something that catches their eye in the "Related" section on the side and watches this video then finds another interesting video on "Related" and watches and so on. This process can last for hours, leading the view to videos completely unrelated to the original video they watched, this is very important to discovery of small videos. It is sometimes very hard to stop once you've started.
Dylan: I'm bored...
PacMan: How about we do some YouTubing?
Dylan: Okay, that sounds fun!