You already learned that 歌謡 means "song," so when you add music to that it becomes a super popular song. It's not just a song anymore. It's in everything you hear!

More broadly, this can refer to a whole genre of Japanese pop music from the Showa era. In other words, it refers to retro J-POP or Showa-era pop music. The word kayokyoku is sometimes used for it in English too.