I have no idea what a bachelor topic is. I know what a Master's thesis is, but a bachelor's doesn't have a thesis. Or does it? Perhaps he was asking about your major.

I think your prof wants to know about your topic of thesis in graduate university.

Originally, the German university system had "Diplom" and "Doktor" as degrees. Both include a thesis ("Diplomarbeit", "Doktorarbeit"). When the "Bacchelors" and "Masters" degrees from the English university system were introduced, many (if not most) universities continued to require a thesis as part of the degree. So the professor was asking about the topic of your final thesis in whatever studies you are doing (probably just to make small talk, because of course every student can talk about something he or she is putting a lot of effort in).