The Passive Volume Kit includes a high-impedance volume pot and other components that help impedance-match your acoustic pickups to your amp, to prevent loss of high frequencies (muffled sound).
The capacitor is used to prevent the loss of higher frequencies as a pot is turned down, it is sometimes accompanied by a series or parallel resistor, to limit the amount of treble being retained and to match it to the pot's taper. They are not necessary in every install.
The Passive Volume Kit Includes:
PD-0103-00 Switchcraft 12B Input Jack
PD-0105-00 2 Meg Volume Pot
PD-0104-00 1 Meg Resistor
PD-0102-00 330 pF Capacitor
PE-5017-00 Summing Board
PE-5021-00 Dual Connector Cable