Every computer contains an internal clock: the clock generator.
It produces the system clock signal (tick) that synchronize the various computer components (device ?).
The clock generator is often changed by computer enthusiasts to control the speed of their:
The clock generator is a circuit on the motherboard.
Typically the programmable clock generator is set by the BIOS at boot time to the value selected by the enthusiast, although some systems have dynamic frequency scaling that frequently re-program the clock generator.