主旋律 literally means 'main melody' in music, referring to a theme or leitmotif that recurs in a composition. Beyond music, it is used metaphorically to denote the dominant or officially promoted theme in society, culture, or politics — for example, 主旋律电影 (main melody films) are state-approved productions that promote socialist core values. You can use it for the central idea of any work, speech, or policy.