The power-on self-test (POST) is the first step in bootstrapping. The POST is essentially a piece of code that the BIOS runs to find out which type of processor is on the motherboard and verifies the amount of RAM. It also identifies buses on the motherboard (and other devices) as well as which devices are available for booting.
The BIOS indicates any system problems that the POST finds by either on- screen display codes or beep codes. For example, a displayed 301 Error would most likely be a keyboard issue. Or one beep might indicate a memory error. This all depends on the type of BIOS used. Some common vendors of BIOS are American Megatrends (AMI) and Phoenix Technologies. Your motherboard should come with documentation about any possible BIOS error codes. If not, the documentation can usually be downloaded from the manufacturer’s website; you just need to know the model number of the board. In the case of a proprietary computer (Dell, HP, and such), you need the model number of the computer to download any necessary documentation from its website.