I think you're referring to POST, or Power On Self Test.
The computer runs through a number of hardware self-checks to ensure it's running at a minimum standard in order to proceed with loading any type of operating system. Typical checks are availability of RAM, video card, keyboard. If any of these are missing or defective, the computer issues a series of beeps. Each manufacturer has its own set of beep codes. You can find these by Googling your motherboard manufacturer's name and the term "beep code".