AC is wibbly-wobbly. It makes a nice up-down curve.
It goes from minus some voltage up to plus some voltage.
So at points it’s far higher than 120V and that’s the peak voltage.
But because it’s up and down all the time, sometimes there’s no voltage at all. When it’s all averaged out, those wibbly wobblies would “flatten” to be at the RMS voltage.
Imagine a series of hills and valleys and then you level them out to one flat surface. The height of the levelled-out land is the RMS voltage. The height of the top of the hill before you start is the peak voltage.
In effect the voltage will vary between +170V and -170V, but the amount of actual power if you average it out over time is about 120V.