Popular, or mass culture is the water in which we swim. It is always around us. Most of us just take it for granted, and think only in terms of our likes and dislikes. We do not think much about that culture as a meaningful place. The closest we might get is to talk about "society", as if it is a force or agency of some sort ("Society is to blame").
Critical theory is a way of approaching contemporary culture without supposing on the one hand that there is some mysterious force that just pushes us around, or on the other that it is just the sum of all our intentional wills.
But the term "popular culture" already should raise questions:
Actually, some figures in critical theory in the narrowest sense (Frankfurt school) were very suspicious of popular culture and the social technology needed to sustain them. Adorno, for example, championed "serious" music over jazz. The Frankfurt School was critical of "the culture industry".
Popular culture has to be seen in its context, its place in a society that values conformity and lack of dissent. A critical view of popular culture involves seeing it as the pacifier of the people, the new opiate of the masses.