The outer circles represent a monochord; they span a few octaves. You can directly change their pitch (in Hertz) by dragging their colored region. Inner circles act as tuning pegs and allow altering the tuning with more precision; spinning a full turn on the middle one will detune the monochord by about a semitone, and the innermost will make really micro-tonal adjustments. Press the button on the middle to enable or disable that monochord, and press the space bar to play the enabled ones. Have fun!