It means absolutely nothing. It is generally used as a slipshod way of referring to people on the middle band of incomes, say £30,000 to £50,000. It includes people of both sexes, all ages, all or no level of education and with every interest and none at all.
DH and I come from entirely different backgrounds, incomes and childhood experiences but we, and our families were joined together by a similarity of attitudes and views, our parents were good friends, although they had different incomes, occupations and life style. Throughout our lives nobody at any time has ever asked either DH or me about our backgrounds in a way that would decide whether we did or did not get a job, join a social organisation or make a social contact. Our friendships are based on shared interests, not on some accident of our friends birth. I discovered quite recently that I was quite mistaken in what I thought I knew about a long-time friends childhood background. As it was irrelevant to our friendship it had never really arisen as a topic of conversation.
I have yet to meet a country that didn't have a hierarchical social system. In this country the idea of class structure, however much it may have existed in the past is now a construct of the media and political classes. I have yet to meet anybody who talked or took any notice of it in normal life.