ce prof = this teacher ce prof-ci = the teacher over there. It means you have several teacher possibly in the same room and you are talking about one very specific one. (they don't have to be in the same room, but it's there to avoid a confusion) If you were talking to me and I got confused with which one you talk about, you'd say "non ce prof-là". "-ci" and "-là" are used for "there". So it's used to pinpoint exactly when there is a risk of confusion.