From my (basic) understanding, it is effectively the average voltage value when you've got an alternating voltage. An alternating voltage is one which goes from positive to negative again and again. Google "sine wave" if you don't know what I'm talking about. You can't just calculate the typical "average" value of this voltage wave, because that would be zero, so rms is a clever mathematical way of getting around that so you get a true measure of the average voltage.