RMS stands for Root Mean Square.

There are two values you want to worry about PEAK and RMS.

Peak is what is says, the PEAK value of the music and you want to keep that just below 0dBFS (for most music) and the RMS or the AVERAGE value of the music and depending on the music can be somewhere in the range of -6dBFS for Hip Hop to maybe -18 to -12 dBFS for classical or acoustic music. A good explanation can be found on Bob Katz's site http://www.digido.com.

Hope this helps.

excellent stuff. thanks. how do i check this in Cubase SX? I presume I should be worrying about while mixing or do i wait until i mix down the stero file and am mastering in Wavelab for example?

Here are two freeware plugins that give you, among other valuable information, RMS levels for your music

download SPAN
http://www.voxengo.com/downloads/

and/or RMS buddy
http://www.macmusic.org/softs/view.php/lang/EN/id/2348/?vRmtQjpAznOhMaS=1

OMTA