RMS (root-mean-square) is just a way of calculating power (or voltage or current) especially when a waveform is non-linear.
It was mainly used back in the 70s when amplifier manufacturers were publishing ridiculous wattage figures and the FCC required a way to reign in the madness and try to get some standardization. What it means in the real world is continous power, not some wacky tweeter blowing peak number with 68% distortion.
It's still pretty much a useless number for anything other than marketing, much like horsepower figures on new cars. But it's still a darned sight better than the measurements from the 70s where it was not unusual to see recievers that made maybe 10w advertised as 100 or even 150 watts.
