The POST test (Power-On Self-Test) is a test that runs on machine startup that sanity checks whether the computer can boot up properly. It checks whether essential hardware components are functioning correctly.
If the test fails, it causes the motherboard to emit some beeps or display an error code on a display on the motherboard to prompt you to troubleshoot. At this point, you’re meant to refer to the motherboard’s manual to determine what the error code or beep sequence means.