When someone posts a video they can set how they want comments to work.  A person can totally disable comments, comments can be freely posted, or comments require approval from the owner of the video.  When set to approval, the comment will be sent to the video's owner, and once he decides it is something he wants on his video he can approve it.  Usually if someone sets their comments on approval, it means they are expecting a bunch of negative feedback and they want to sensor it.  The disabled comments option is pretty much the same thing, except it means the video owner is also lazy and doesn't want to bother with the approval process.  I admit I have used all 3 commenting types for my videos, hehe.