Refers to the concept of the "dream, within a dream, within a dream" from the movie Inception; except users are experiencing a video, within a video, within a video.
Can often be a great way of finding obscure and random YouTube content that usually ends up being a world away from the initial search.
"Yeah, how'd you get to that?"
"Tubeception, man: I started on a Slayer video, then just clicked away until I ended up at see-saw kid!"
"I was so bored at home, I ended up having a Tubeception session and realised how many damn cats are on here!"