No, I mean like, say you're watching a video, but you want to keep browsing YT or surfing the web without leaving the video or opening new tabs/windows - you could "pop out" the video you're watching, and the video itself would relaunch itself in its own separate little pop-up window like a media player: no comments, no suggestions, just the video itself in its own self-contained window sized to the video player. Matter of fact, I managed to find a screencap on Google to show you what I'm talking about:
It did not last long at all; I believe they first implemented it sometime in late 2007, but I don't remember it lasting long into 2008 at all. But, man, it was a very useful feature indeed.