SKB FN-8 FootNote Amplified Pedalboard Aux input used for mp3 player (iPod, etc.) for accompaniment or other type play along situations Guitar/Pedals input to either plug in your daisy chained pedals or plug your guitar straight into the Footnote Volume, Bass and Treble controls are the same as you would expect on any small combo amp and function the same way Cable tester tests your  audio cables to ensure good audio cables are used throughout your daisy chained pedals Line Out to be used with a standard  instrument cable for a high impedance connection or plug into a DI box for low impedance connection to a mixing board or recording desk