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The designation and physical unit Hertz (abbreviation or shorthand version: Hz) stands for a frequency that denotes a number of oscillations in one second.
In computer systems, the operating clock or the clock frequency of a Processor or the frequencies of bus systems are specified in hertz.
In the case of monitorss, it is used to specify various frequency parameters that essentially reflect the quality and performance of the display on such a device.

CPU MOS 6510 of a C64.

Since the operating frequency of processors is relatively high nowadays, a multiple of the unit hertz is used. The first home computers had processors with a few megahertz (abbreviation: MHz, million oscillations per second), while today's computers work with processors in the gigahertz range (abbreviation: GHz, billion oscillations per second).
In monitors, the value range of the refresh rate (or vertical frequency) is in the simple hertz range (a measure of flicker). The line frequency (or horizontal frequency) in kilohertz (abbreviation: kHz, thousand oscillations per second; colloquial, but scientifically incorrect abbreviation also as KHz) is considered a measure of vertical resolution and the specification of the video bandwidth, which is already in the MHz range, is directly proportional to the possible resolution (tolerable, but not necessarily cleanly displayable pixel number) of the screen.

For comparison: The Commodore 64 has a processor clock of about 1 MHz. A C128 operates at about 1 or 2 MHz, while the original PC from IBM already operates at a clock rate of about 4.77 MHz. With the so-called "turbo button", the clock frequency of PCs with processors 80286 to 80486 could be halved.

Typical monitors for the C64, C128, etc. are in the region of about 15 kHz for the line frequency, while standard VGA monitors in the PC area already penetrate the 35 kHz range, if an ergonomic refresh rate is assumed.

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WP-W11.png Wikipedia: Hertz