It stands for Root Mean Square.
Basically the max output for a speaker is worthless.
If you have an stereo that has 200 Watt max output then it can hit 200 Watts, but most of the time it will operate at a much lower level.
RMS is the average mantained output (basically) and is a much better indicator of performance.
I've seen a bunch of car stereos that boast 50 watts, but only have an RMS value of about 19!
But hey, you also have to check out the signal to noise ratio... that is VERY important
...ah to be an EE again...
HTH,
-Ryan