I am not sure I quite understand what they mean when they say that; but what I take them to mean is that they have an opinion on a whole package of issues. Taxes, immigration, abortion, welfare, firearms, gays, transsexuals, women's rights, race, free speech, climate, christianity, islam, capital punishment, and so on. Which is surprising to me, because I don't have opinions on most of these topics, or have opinions that are so mild that they can be flipped one way or the other depending on the persuasiveness of the argument. I do have opinions on the things that concern me directly; which would be the freedom of speech (yes please) or organised religions (no please), or maybe taxes (I'm a greedy bastard, not that I have much to be greedy about); but apparently there are many people who feel strongly, very strongly on a much wider range of topics that don't immediately concern them.
I wonder when and how people acquire those strong opinions on things and decide that they are "progressive" or "on the left". Is it college? School? Social media? Peer groups?