You click the buttons to change the Nyan Cat in the center and to play the Nyan cat music (Or play the Robotic Nyan Cat music). Do not touch a button while another is already playing its music otherwise the game will break.
Please do not insult my stars, I'm just not that good at drawing on a computer. Also, if you want, stare at the Nyan Cat in the middle of the screen for a very long time without pressing any of the buttons. It'll be cool but I won't spoil the surprise.