This topic is quite a simple one. NOOP stands for no operation. It takes no argument and does nothing. Just because I'm a nice person, here's the code for the NOOP command in the Client::handle_cmd method:
Command::NoOp => send_cmd(&mut self.stream, ResultCode::Ok, "Doing nothing..."),
Yes, I know, you're amazed by such wonderful code. But don't worry, you'll able to write something as good as this when you grow older!
It's now time to implement the next command!