what the cap does is act as a fast recharging battery, when your bass hits, the power is stored close to the amp in the trunk, it will provide the power the amp needs to slam the bass through your speakers, and then it will recharge very rapidly while the bass is on a 'break'
what this does is like i said keeps more voltage near the amp instead of having to draw it all the way from the battery in the front of the car
yea the cap still draws it from the front however it does it at a controlled rate, much smoother and slower than the amp requires it to be drawn
it is made of two pieces of metal that are placed close together but not touching, they were made for A/C to D/C applications, a/c power can pass between the two pieces of metal but D/C power can not, there for it 'filters' out any D/C power noise (basically)
that of course is not the intent in car stereo.
the larger the cap the more power you will have readily available, I had a 10F cap in my car i was running a 1200W mono amp for my sub and a 1200W 4CH for the components, it still drained fairly quickly, it all depends on the volume you intend and the type of music, for rap music that hits hard but not very fast you won't need as much, but for like heavy metal with the drums and the bass guitar you will have more rapid bass and the cap will drain much faster.
caps are fairly cheap too, don't go to bestybuy or any walk in store, I paid 130 roughly for my 10F and that is the same price you will pay for a 1 yea ONE farad at the store if you walk in, I have seen up to 50 i think, maybe more, you can even hook up multiple caps together for more power, but if that is done wrong you will lower the power of the system
keep all this in mind but remember that a cap will not help a weak battery or weak alt, if your lights dim the cap may or may not help, If you drive an FC, get an FD alt and even rebuild it for more than 100A, get an optima yellow top as well, these things will help your car in many more ways than just the system...