Since I've got a few minutes to burn, hopefully you can follow all this...

*puts EE hat on*

RMS is really sort of like a formula, it stands for root mean square. See, when you have a power output, it is not constant. It will be outputted in the form of a sine wave if you look at it in the time domain (power on the y axis and time on the x axis). Here is an oscilloscope image of a sine wave for example, disregard the bottom input, look only at the top (yes, I am a nerd and using my own pictures h: )




Now, usually the power output signal would be centered, if you will, on the x-axis, meaning the average value of the signal would be 0. Just imagine that. Now, to find the RMS value of this signal, we must follow the 'formula'. First, we square the signal. Then, find the average value of the signal, using an intergral (oooo, calculus...), and finally we take the square root of that value. The resulting value is the Root Mean Square, or RMS, value of the signal

It is just a universal way of putting a constant value on a non-constant signal.

Now, when companies use 'max power' ratings, this is the maximum value the load will see at a certain point in time, referring back to the sine wave. This value is misleading, because your speaker will only see this value for a fraction of a second, where as RMS value is for an infinite time.