Meaning, almost every Ghanaian has this last name. It is the equivalent to names like "Smith" for whites, "Chan" for Asians, and "Singh" for Indians.
If you know a Ghanaian, you know at least one Mensah.
Girl 2: Which Mensah?
Girl 1: Peter Mensah of course. Which Mensah did you think I was talking about?
Girl 2: Well, there are a lot of Mensahs. It is the "Smith" of Ghanaian names. Be more specific next time.