a capacitor serves as a sort of reserve. Ever seen a car w/subs that when the bass hits the headlights dim? a cars electrical system is only set up to supply so many amps of current. A capacitor stores up energy so when that big bass note hits and your amp needs that xtra kick of current NOW it's there and doesn't pull current the car would otherwise use for its headlights, tail lights, ect.
And spade is right, if you are going to buy an amp focus on the RMS rating since that's the amount of power your speakers continually get from the amp.
