This is where you can set the Parametric part of the EQ. You're on the center frequency for the bass frequencies. If you press left or right you'll adjust what the center frequency is for the bass level. If you press up or down it will adjust the Q or how much around the center frequency it adjusts. If you press the audio button again and again you'll cycle between the bass, midrange, and treble frequencies. Most Pioneer head units have had this adjustment for quite awhile.