The ideas, customs, art, and entertainment that are widely shared in a society are popular culture. Today, popular culture includes the TV shows, music, and online videos all your friends are talking about.
For sociologists, popular culture includes practices and beliefs that the whole society shares. Most people think of it in terms of mass media — movies, magazines, social media, and other entertainment sources which are so widespread that they saturate the culture. Even if you never watched Game of Thrones, you know all about it, because it's part of popular culture. This accessible kind of culture is often contrasted with more elite offerings like Shakespeare, the opera, or Renaissance art.