Root Means Square is a way of representing a value that is constantly changing between two values; eg. an AC voltage.
In Sinewaves, the RMS voltage is the maximum amplitude multiplied by the square root of 2.
In the blue trace, the RMS voltage in this circuit would be measured at 120V, but the actual AC voltage changes from +170 to -170V. The voltage amplitude is 170V, and the peak to peak voltage is twice that, 340V. if you measured AC volts on a meter, it would show the RMS voltage of 120V instead of flicking between +170 and -170 sixty times a second.
there's all sorts of other calculations for different waveforms as well, go look at Wikipedia.
