what are those cylinders usually located where subwoffers would be that keep from the subwoofers killing your battery or taking over all the power

A capacitor. Most of the time people throw them on there car as a band aid to diming lights. Can be helpful on a wells built sound system. You will receive aruguments that they work and they don't.

oh well i thought it had some other name than a capacitor. but i was just thinking it slowed down or stoped the process of draining a battery using the system when car is off. i know my headlights dim a little bit while bumping

its just keeps constant power to your amp.
and if you need one it just depends on how many watts your runing.
im runing 1400 in my ej6 with no cap, but i do have a upgraded battery.

The battery only supplies current while the car is off. When it is running, the car gets its power from the alternator.

personal opinion- caps are junk.. ok say you pay retail and get a 1 farad cap with a digital top, est retail on a brand name cap $120 ish or more, at this point you could have gotten an xstatic batcap- which your factory alt can charge fine, and you would be alot better off. my batcap 800's i had can deliver a rated 800 cca for 30 seconds until depleted discharge.. not that you would need that much, but it can do it..