I am looking at a log and I am trying to wrap my head around the root mean squared value...I have look up defenitions but I still cannot get my head wrapped around it.

For instance, if I am looking at a log that is showing values from 0 - 100

The max lateral acceleration is 40

The average is 20

The log shows the rms value being around 13..but how do you come up with that figure?

What exactly is root mean squared and why is it important?