It’s a reference to the sociological concept of marked versus unmarked identities. If you think of an American you probably think first of a white man of Christian heritage. If you think of a Chinese person you picture someone of Han ethnicity, not Miao or Manchu. If you think of a nurse you picture a woman. All of the people who fit that identity but are different from the unmarked version are marked by their difference from the unmarked version that springs to mind unbidden when the concept is named.