Speakers use electricity from you battery to create sound.
What a cap does it when the amp requests a large amount of power (usually for low bass notes) sometimes it takes more then the batt can supply in the amount of time the amp needs it. So your low bass notes are not as full as they could be because of lack of power.
A cap holds some energy in it for this and releases it so the amp is never starved for energy and thus your bass responses are fully developed.
hope this helps.
Oh here for a Professional responce...
look here