"POST" stands for Power On Self Test. When the computer starts to run, it does a self test of the components on the motherboard. This board has an onboard POST codes display LED as the below picture. It helps user easier to get to know if the computer's boot process properly or not. It also helps user easier to figure out what the possible failure might be if the BIOS detects a problem and cannot go ahead. Here is POST codes checkpoints for you reference.
