What is 50HZ AC?

AC is alternating current. Or the voltage output changes from + to - USA uses 120 V AC 60 HZ as the normal plug power. Or the 120 V positive will go negative then positive every 1/60 seconds. Or the change will happen 60 times per second. Japan Uses 110V 50 Hz. That why a lot of the AC appliances that can be used in Japan or the US will be marked for both.