What he posted was a mobile youtube link that would allow a mobile user to automatically sign in from their iphone, andriod or whatever. it was deveoped originally as a time saver for mobile device users and only shows your own account to you.
It would show either a few random videos to click on and a "sign in"
icon on the top right, or, if whoever used your computer to access their account and saved thier password on your machine.
Not really a security risk unless...... you posted to youtube on another machine, saved the password and didn't wipe the cookies off. Theoretically the next user could pull up your account using that link.
Kind of unnerving and not really cool freaking people out like that but relatively harmless.