A specific span of time in which something happens. When the period changes, you’re adjusting the frequency at which something will occur.

Period is the frequency at which a pulsed signal is sent to an electronic device. For hobby servos, this period required is typically 20 milliseconds (ms) which means it requires that a signal be sent 1000 times per second. The period is also commonly measured in Hertz (Hz). Hz is cycles per second so a signal of 50Hz is 50 cycles per second which equates to 20 milliseconds.