Hearths are the "origin" of a certain culture, be it music, sports, television, etc., they all begin somewhere, and these places are called a hearth. Generally, a certain form of popular culture originates from this location, and then uses contagious diffusion to gradually spread to people who are willing to listen to the music, watch the show, etc. From there, that idea is spread frmo person to person to become well known by even more people.