When the computer is turned on, the POST program tells all devices to run their built-in diagnostic. The diagnostic result can be relayed in beep codes, text messages or as LED on physical cards.
Beep Codes
Certain beeping noises will play depending on device status No beeps: Motherboard problem 2 short beeps: All is well Long beep followed by 2 or 3 short beeps: Missing video / Motherboard problem Indefinitely long beep or strange beep: Missing RAM
Text Codes
These vary depending on operating system. It will just tell you the error flat out.
POST Cards
If sound card or graphics card is dead, then you have no other option but to mount a POST led.
It will display a 2-character post code to tell you the issue
USB POST cards
Like regular post expansion cards, but they can be connected via USB