A capacitor is a passive (it consumes but does not produce energy) two terminal electrical component. In building a sound system for your car, you would want to use a capacitor for a couple of different reasons.

The main reason a capacitor is recommended is for power reserve. The capacitor stores energy and releases it very quickly to the amplifier(s). How does this help? Music is dynamic, no matter what type you listen to. The dynamics in musical content can vary from soft voices to thundering bass notes. These moments are when the benefit of having a capacitor is heard. The amplifier requires a constant amount of steady power to function efficiently and power the speakers. A capacitor will keep the power going to the amplifier consistent and stabilized, which means a better sounding system. Important: The remote output of the head unit is connected to the remote turn on of the capacitor. The capacitor will not function if the remote wire is not connected to your head unit.