RMS = root mean square
it refers to the 'true'/'continuous' power as opposed to peak. you'll see an amp rated at 1000W (most likely that's going to be peak power since it's greater - marketing hype) the rms value is half that, in this case the true power would be 500W
it comes from the equation:
1) P=VI
2) V=IR
thus P=I^2 * R
where P=power; V=Voltage; R=Resistance (load) and I^2 means "I squared"
A) P=Irms^2 * R --> true/continuous power
B) P=Ipeak^2 * R --> peak power
C) Irms=Ipeak/sqrt(2)
sqrt(2) --> "means square root of 2"
put C in A -> P=Ipeak^2 / sqrt(2)^2 * R
simplifying --> P=Ipeak^2 * R / 2
clear as mud?
wikipedia explains it in more detail