Complex sentence: Main gameplay is formed around the simple idea that the player must press various buttons at the correct moment, which is indicated by the music and the notes which scroll down in a tune of the music track visualized on screen.
Split into simpler sentences: Main gameplay is formed around the simple idea that the player must press various buttons at the correct moment. Moment is indicated by the music and the notes which scroll down in a tune of the music track visualized on screen.