In alternating currents, the current signal "cycles" from -170 volts to +170 volts in a typical North American 115 volt circuit you will find in common households. If you show the voltage of the electrical signal over time on a chart, you will see a rolling wave. One full cycle will start at 0 volts, go to +170 volts, go to -170 volts, and then back to 0 volts (where the pattern repeats). Another way to figure out what part of the signal belongs to one cycle is to start at zero and end at zero and this whole section of the signal that you have selected should be able to be repeated. If repeating your selection doesn't reproduce the original electrical signal, then you selected the too much or too little of the original signal.
Cycles are measured in Hertz (Hz). In North American 120 volt circuits, a cycle represents 16.67 milliseconds of time, because that is 1/60th of a second. So, North American circuits are 60 Hz or 60 cycles in a second.
The reason these circuits are 120 volts, instead of 170 volts, is because you have to take the average voltage of the electrical signal. Since the voltage is alternating, you have to average out its value.