What is POST in terms of BIOS?
When a PC is turned on, the first thing the BIOS does is Power On Self Test (POST). At the name indicated, POST is a type of test that is primarily used to diagnose that the BIOS and hardware is running smoothly. Specifically it tests the computer's basic input/output system ability to communicate and control the computer keyboard, random access memory, disk drives, and other hardware.
If the test finds that the BIOS is functioning optimally, then it goes ahead and boots the computer. If it is not, then an error message is displayed on the screen.