If the output of an ac generator is connected to a resistor, an alternating current will flow. An ac generator generates a voltage and an EMF with the shape of a sine or cosine curve.
The power is given by the product of voltage and current, so the power will have the shape of acurve.
The RMS voltage is given by the square root of the average of the voltage squared:
Likewise the RMS current is given by the square root of the average of the current squared:
The RMS power is given by the product of the RMS voltage and the RMS current:
When ac values for current, voltage or power are being quoted, they are the rms values.