There are many things that would be studied by people in the discipline of popular culture.
Such scholars would study different genres of popular culture. They might study, for example, the popular cultures of different ethnic groups in the United States. They might study the popular cultures of different age groups or different regions. They might study television or music or literature. All of these are different genres of popular culture.
Such scholars might also study various topics in popular culture. For example, they might study the impact of popular culture on things like teen sex or violence. They might study the ways in which popular culture contributes to (or obstructs) the process of assimilating people of various racial or ethnic groups into "mainstream" American society.
In short, anything that is popular in our culture might be studied. We might look at the things that have affected the popular culture and the way that popular culture affects our society as a whole.