RMS means "Root Mean Square". It is calculated by using Calculus or by an approximation. So the value depends on the waveform. I'll post examples later.
An example of why a tech may need to know the RMS is:-
The RMS value allows you to calulate the power dissipation in a resistor.
When a wave form has an RMS value of X volts, the heating effect is the same as X volts DC. As the heating effect is proportional to V squared, you have to first square the voltage then find their average then square root this value.
Frank