When IBM released there first IBM PC, they included a safety features that had never been seen in a personal computer,one of this features is the POST. POST is a series of program routines buried in the motherboard ROM-BIOS that tests all the main system components at power-on time. This program series causes the delay when you turn on an IBM-compatible system;the POST is executed before the computer loads the operating system.
Whenever you start up your computer, it automatically performs a series of tests that check the primary components in your system. Items such as CPU, ROM, motherboard support circuitry, memory, and major peripherals are tested. These tests are brief and not very thorough compared with other disk based diagnostics. The POST process provides error or warning messages whenever a faulty component is encountered.It is the first line of defense on your PC,if the POST encounters a problem severe enough to keep the system from operating properly, it halts bootup of the system and produces an error messages(ex. unreliable HDD, memory error, FDD error or sound beeps).