Voltage is electrical potential, so Vrms stands for “root mean square” voltage. It’s a way of measuring how much the voltage changes with time over some interval. Voltage is typically measured as volts (or millivolts or kilovolts) from a given reference point to ground.
Vrms stands for ‘square root of the average value of a signal, taken over an interval’ and it can be calculated by using this formula:
For example, if you want to measure the degree of variation in 1000 ms intervals (0.001 seconds), then 1/1000×60 = 0.0000666…