Polyphonic describes music where several tunes, or several independent lines of notes, are sung or played at the same time, and each line could almost stand on its own. A round like “Row, Row, Row Your Boat” is a simple picture of this, and a lot of old church choir music is the classic case. It is not the same as one singer with chords or a band underneath — that is still mainly one tune, even if it sounds full. The word belongs to music class, choir, and writing about classical music; pop songs are rarely called polyphonic. Though actually, people have stretched it: old mobile phones sold “polyphonic ringtones” just to mean several notes at once instead of a single beep. That shop use is looser, but it is the same “many sounds at once” idea.