it means subscribe to someone else's youtube account. You will then receive a notification every time they publish a new vid. People try to get more subscribers to their accounts.
Someone you throw paper airplanes at, perform as a Tourette's child for, compliment on her dress and say you have one just like it (if you are male), and generally confuse and irritate. The ultimate goal is to either make the substitute break down laughing or reveal his/her shocking brain-dead-ness by convincing him/her that you really do talk like that.
"Wow, that sub we had in history was wearing a belt and suspenders."