Frequency rates are commonly expressed as units of "hertz" which represents cycles per second and is abbreviated as "Hz". One hertz is the number of times that a pattern - such as a digital pulse or analog sound wave - repeats each second.
One hertz, or Hz, means that the frequency is one cycle per second. Kilohertz, or kHz, refers to how many thousands of cycles occur per second. Megahertz, or MHz, refers to millions, gigahertz, or GHz, refers to billions and terahertz, or THz refers to trillions of cycles per second.
Consider a LED that blinks. One full blink cycle contains both the on and the off state. An LED that blinks at one Hz would blink on and off every second. An LED that blinks at two Hz would blink twice each second, while an LED that blinks at 0.5 Hz would blink every two seconds. An LED that blinks at 1 kHz would blink a thousand times a second - so fast that you would even notice that it was blinking.
When building robots, you may be concerned about the speed of the microcontroller's clock ticks, the frequency rate of a PWM signal, or the pitch of a tone being sent to a speaker. All of these frequency rates will typically be measured in hertz or one of its multipliers like kilohertz or megahertz.