POST is an acronym for Power On Self Test. This is a series of diagnostic tests that a computer or computerized electrical device runs whenever it is switched on or restarted.
A Power On Self Test happens as part of the start up sequence when a system’s Basic Input/Output System (BIOS) starts. It checks that hardware, such as the keyboard, memory, processor and disk drives, are installed and working properly. If the POST completes successfully, the computer continues to boot and runs the operating system and software.
The BIOS issues a warning when hardware is missing or is found to be faulty during the POST. Often this is a series of beeps, but it can be shown by flashing LEDs or an error message on a display. These are frequently fatal warnings that will stop the system from booting.