Sensei means teacher or generally speaking someone strong enough to be your teacher. It is a form of respect. It can also refer to an older player who used to be stronger (than the one calling him Sensei).
Sidenote: in HikaruNoGo, an older player comes into the club to find 11-year old Akira, a kid aspiring to become pro. The man asks if Akira Sensei is present. I can also imagine that Yoda Norimoto will call Sakata "Sensei", although Yoda is probably stronger at the moment.
Scartol: Let's talk about what it means to be a sensei.
The -sensei ending is one example of a Japanese name suffix.