What is the role of a rms voltmeter?

Answer 1

An ordinary voltmeter reads average value of the voltage input. However, the "heating" power of a voltage is indicated by the root-mean-square (rms) value of the voltage. So if you are interested in the real energy value of the voltage, you need to know the rms reading. For a sine wave, there is a constant relation between average and rms value, so voltmeters can be calibrated for sine-wave rms. However, for complex waveforms the voltmeter must measure or calculate true rms.

See http://en.wikipedia.org/wiki/Root_mean_square

Answer 2

Almost all AC voltmeters are rms voltmeters. Any time you measure an AC voltage it will probably be rms if you used a voltmeter. Or probably peak-to-peak if you used an oscilloscope.
If you read the peaks off the scope you divide by 2.8 to get the rms value (or heating equivalent) IF the AC is a sine wave. If it is not a sine wave the calculation is more complicated.
Most AC voltmeters use a DC magnetic movement with a rectifier and a smoothing capacitor, so that it really measures peak values like the scope, and the scale is corrected like the scope for sine waves. If the AC wave is not a sine wave you are out of luck. You read a number but it is somewhat inaccurate.
UNLESS you get a voltmeter marked "true rms". These types are more complex, a bit more expensive, and do the work for you if you want the true "heating equivalent" of an AC wave with a funny shape to it (distorted, square, triangle, audio, etc.).