POST (Power-On Self Test) codes are generated by the BIOS/EFI during the power-up sequence to indicate which hardware is being initialized at that moment. Many systems display POST codes on the screen, or via a small array of LEDs or a pair of 7-segment displays mounted on the motherboard, or as a series of beeps..
For motherboards on which POST codes are brought out to a system bus as well, the code can be displayed on a special expansion card often called a "POST card". Many POST cards record the entire series of codes generated during power-up, including special codes that indicate non-fatal errors.
In most cases, the last POST code displayed (or sounded in beeps) reveals the stage at which the hardware initialization failed, indicating which component must be replaced. These may include RAM modules, video cards, other expansion cards, a variety of onboard devices, etc.