RMS stands for Root Mean Squared it is the equivlent to the DC power in an AC circut.
example an AC circut is one that has an on and off time. because of the nature of a sine wave like in audio it is always in a state of on and off. CD or power from a battery is always on constant in a compleate circut.
with audio we refer to the RMS value as the average time the current or voltage is present between the on and off times.
in simpler terms RMS = .707 of peak.
if you have an AC signal of 100 watts peak it is equal to 200 watts peak to peak and 70.7 RMS