As John said, RMS is more or less the average voltage (volume) level of the voltage levels measured over a sequential series of samples.
It's not a true mathematical average; the calculation for RMS is mathematically different than a simple average, but the concept is basically the same.
The purpose for it - in this subject, anyway, - is that it more closely resembles the volume as perceived by the human ear than simple peak volume level measurements do. For example, something with a peak level of -3dbFS and an RMS level of -10dBFS will actually usually be perceived as louder than something with a higher peak level of 0dBFS but an RMS of only -13dBFS.
G.