Hertz is a unit of frequency. It represents the amount of times an event repeats in one second. We hear it commonly on the specifications of computers, where currently it’s mentioned as MHz (MegaHertz) or GHz (GigaHertz) and actually gives the frequency of the internal clock. In stroboscopic photography, it indicates how many times per second the flash will be fired. This factor, combined with the amount of repetitions gives the total duration of the burst: Duration = Repetitions/Hz.

For this example we used a metronome to illustrate this apparently complex, but actually quite simple terminology. The metronome was set at 1 second ticks.
For image 1, we set the flash to fire 5 times at 5 Hz: 5 times/5Hz = 1 second duration.
Image 2, 10 times, 10 Hz = 1 second duration
Now, for Image 3, we doubled the amount of times to 20 but increased the Hz 5 times to 100, resulting in an interval of 20:100 = 0.2 secs.