Why RMS?
As most AC signals are symmetrical around zero, their mean value is zero.
As 'a minus times a minus is a plus' the process of squaring a negative value then finding its root makes it positive.
So RMS effectively means 'flipping' the part of a waveform below the zero live and then averaging all the values.
If this is reminiscent of calculus, that is because it is. Theoretical RMS values are easily calculated.
True RMS values are more tricky to measure. Most meters assume a sine wave. True rms meters are more complex and costly, although for audio signals a good moving iron meter will do the job.