In FNF Fazbear Fight: Freddy vs Purple Guy, you play Purple Guy and try to beat the evil robot with just one song.
When the floating arrow symbols above your character on the right line up, that's when you need to press the same arrow keys to hit your notes. If you can keep doing this until the song is over, you win, but if you miss multiple notes in a row, you lose.
Have fun and good luck!