I'm certainly not an expert, but this is my understanding of it.
mods like the Zmax and the VAMO use pulse width modulation to output their electrical signals. For example, if you set your voltage to 3.7V the unit doesn't fire a constant 3.7V while you press the button; instead, the coil is turned on and off extremely fast. The resulting wave oscillates from 0V up to say 4.5V, but because the signal is turning on and off rapidly it averages out to 3.7V. As you turn the voltage setting up the
mod doesn't output a higher voltage to the coil; instead, it increases the percentage of the time that the signal is turned on, and because it is turned on more often the average voltage goes up (and vice versa if you turn down the voltage setting).
RMS and mean are simply two different ways to calculate the average output of that fluctuating signal. RMS is a more complex calculation but it more accurately reflects the setting that you select. If you use a mean calculation the output voltage may be higher than your desired voltage, expecially at low settings. That can be a problem, because the Zmax might actually be sending more voltage to your coil than you want it to.
Because of the confusion with these settings some new
mods like the VAMO V3 have removed the mean option altogether.
Hopefully that helps a little. I'm sure if I got something wrong (likely) someone here will correct me.