If you'd pop into a match with ZITHER!, hopefully you'll see what I'm talking about here.
ZITHER! has three chords (Ice, Fire, and Electric). Those are the Blue, Red, and Purple buttons respectively. (I'm not even sure you can move them around because I haven't bothered to try.) The fourth button 'Song of Magic,' or w/e it's properly called, is also there.
In a match, there is a bar above your health bar which displays your active Chords and Song. To play a song with 3 Electric chords, you fill the three chord slots in that bar with Electric-Electric-Electric and the name of the song should display between that bar and your health bar. (I play on PC, so the display might be slightly different?)
You may have noticed that ZITHER! has skills and passives (triangles and circles) in his ability trees like the other classes. Instead of putting them on the bar, the skills have chord combinations on the tooltips. To play a song, you just have to punch in the correct three chords and hit the 'Song' button. For instance, even if you know that E-E-F is a valid chord combination for one of the songs, if you haven't put the point in on that ability yet, ZITHER! cannot play E-E-F and you will get an 'invalid song' warning should you try to play an unlearned song.
He starts out with the four buttons (three chords and one song) and three songs:
F-F-F which does damage.
I-I-I which provides a small amount of barrier with each application
E-E-E which provides a movement speed buff.
Hope this helps!