As I understand it RMS or "root mean square" level simply means the overall level (as opposed to levels for individual frequencies, known as spectrum levels, which will differ).
Many sound level measuring devices are capable of sweeping through a range of frequencies and measuring the level of specific frequencies (spectrum analysis). When the overall level is measuerd (all frequencies together) this is called the RMS level.
I don't think it's anything you need to worry about, I guess in most manuals and articles relating to recording gear etc. it's just a fancy way of saying 'overall level'.
This is about as technical as I get. I'm sure one of the mastering guys would know more about this stuff.