When referring to speakers, RMS stands for root mean square. It is a method used to calculate the power output of speakers for a given amount of time.
The two power handling ratings of a speaker, RMS or continuous and peak or max, are thermal ratings measured in watts. This refers to the amount of heat that can be handled by a speaker's coil without causing internal damage. A speaker's power output is determined by the type of voltages used. Peak power is derived from peak voltages. If RMS voltages are used, the resulting power is called average power, which is often incorrectly referred to as RMS power. The RMS yields the mean value of how much power an alternating signal dissipates.