it act like a battery that stores energy and when ur amp needs extra energy to hit a low bass note it releases the power. it also makes the notes "hit harder".
its used for subs...
i reccommend not getting a cap. unless you upgrade your alternator in your car because your alternator runs ur electrcal components of your car and the cap. just put extra striain on your alternator and if your alternator doesnt have enough amps your headlights will dim