"단" simpley refers to "Grade/ Rank/ Rate" There is a similar term like "(등)급" Ex) 너는 태권도가 몇 단이니? = What rank are you in Taekwondo?

단(段) is the rank of the traditional board game Go(바둑) There are 9 ranks from 1 to 9 단 for professional player of Go. 9 단 is the highest rank of professional player of Go. Thus, 9단 means "master of some field" 주부9단 means "master of housewife"