Popular culture is not easily defined because there will be disagreements as to where popular culture ends and high culture begins. The basic idea, though, is that there is a difference between popular culture and high culture. Popular culture can be defined as those aspects of culture that appeal to the masses, rather than to some more educated and "refined" elite. We would typically say that reality shows on television are part of popular culture while Shakespeare plays are part of high culture. We would say that rock music or rap music or country music are parts of popular culture while Beethoven and Mozart are part of high culture.
Of course, this difference is very difficult to outline in an exact manner. There are some types of culture (the "Three Tenors" for example) who seem to bridge popular culture and high culture. However, the basic idea is that popular culture is the culture of the masses rather than the culture of the elites.