like a farad cap. Those are good when you are running a lot of power. If you have an amp that is that powerful then you should probably get one because it stores power so that when the music calls for it it can deliver enough power to your subs without draining from the cars electrical system. You can see this in cars that have high wattage systems when the bass hits hard the lights dim inside and out. I am not sure about xenon though. I have never seen it dim before.