One of the choices you have when selecting a multimeter is whether to select a true-RMS or non-RMS multimeter. To understand the difference we first need to understand what RMS represents. The Root-Mean-Square (RMS) value of any a/c voltage or current is its effective or power-producing value. An ac waveform is in a constant state of change. It is constantly changing its value from zero, to a positive-peak value, back to zero, to a negative-peak value, and back to zero.
On a 60 hertz power supply, this happens 60 times per second. So, for every 1/60th of a second, a standard 120 V ac waveform will go from zero, to 170 V, back to zero, then to -170 V, and then back to zero. However, its RMS value, or effective value, is 120 V.