RMS value means root mean square value. It is 0.707 times peak value of voltage or current. The ammeter and voltmeter records this rms value. RMS value for sine wave is more than average value. RMS value is also called effective value.
RMS value of AC is given by DC which is flowing through a resistance for a given time and producing same amount of heat as produce by AC when AC is flowing through the same resistance for same time.