"POST"

Well its not html form submission type that we are talking about,we are talking about what's called Power On Self Testing.

What does it do?
Its basically is used in the process of loading operating system. This is the critical part of the booting process wherein this is held responsible for checking  hardware of the system, so in case anything goes wrong in testing process, it means that there is some part in the hardware that is not functioning as expected and since hardware is the basic part of any system, it makes no sense at all to boot the operating system(OS cant do anything without hardware). This failure makes the process(POST) not to transfer the control over to process responsible for operating system booting. Hence OS does not get booted. You can listen to some noise when any error is observed by POST.

The language responsible for post is fourth, i mean most of the POST programs are written by making use of programming language called Fourth.