What does RMS mean relating to car speakers?

RMS= Root Mean Square.

It means the speakers can take a peak power of 200W for loud bursts witout damage. And can also operate all day at the 100 W Root mean squared power level. (Easy to look it up for a full description of RMS). It also means you will need at least 100 W RMS/ 12 V = 8 amps minimum to power just the speakers.

Don’t worry. They speakers will be loud enough to make your ears bleed if that’s what you’re looking for.

RMS would be the steady-state power rating the speakers could handle. The peak of 200W means it can handle the bass thumps, if that’s what you want.

100 watts RMS inside of a car will perforate your eardrums damn quick.

However, you want to make sure your amp is rated at 100 watts RMS or higher, as the easiest way to blow a speaker is to underamp it. When the output clips, the transients destroy your tweeter cones.

You are correct that there is a 200W max in the system you mention, but that really is not much unless you are installing it in the cab of a pickup truck or only care what it sounds like inside and are not out to impress the neighbors. Trust me when I say that 135W/channel times 4 channels (540W total) doesn’t sound all that impressive ten feet away, but will deafen anybody sitting in the car possibly for a while, if not permanently.

To add on, a 300W RMS system will beat the crap out of a 500W peak system anytime, anywhere, anyhow. While peak numbers are impressive, what you really want is sustainable output unless the only thing you care about is bass thumps. In other words, the RMS number is actually more important for most setups.

Another thing to bear in mind is matching the speakers to the setup. Like @IchtheosaurusRex says, transients kill. The reason many lightbulbs burn out is the thermal stress of going from ambient temperature to operating temperature; a transient. While it’s a bit different for speakers, the fundamental theory still applies, so don’t buy speakers too big for your amp.

Thanks for all the answers guys, they really help!