What exactly does the "main melody" in TV dramas and movies mean?

Especially China's films promoting national ideology. "Main melody" movies refer to movies with major themes of revolutionary history that can fully embody mainstream ideology and movies with realistic themes that are close to the lives of ordinary audiences, promote mainstream values and eulogize human life. Under the guidance of building Socialism with Chinese characteristics Theory and the Party's basic line, we should vigorously advocate all ideas and spirits that are conducive to patriotism, collectivism and socialism, reform, opening up and modernization, provide all ideas and spirits that are conducive to national unity, social progress and people's happiness, and strive for a better life through honest labor.