In audio circles, RMS is an abbreviation for Root Mean Squared (mathemetical fomulae found therein) -- this term is used in many other fields, but the audio usage is taken as the power value limited by the established harmonic distortions, and is often used to describe nominal continuous load of a component or system measured in wattage. In the case of Head Units and Amplifiers it is the amount output'ed, and for speakers/drivers it is the amount that can be safely input'ed for long periods.

This is different from the other measurement used, which is Watts per Channel, or WPC, that is generally the maximum output possible in a small burst, but not continuous service.