The BIOS (Basic Input / Output System) is a chip on the computer's motherboard that is designed to provide the interface required between hardware devices such as hard disks, display and the software usually operating system. 
The BIOS has instructions as to how to load the basic computer hardware along with a test usually referred to as a POST (Power On Self Test) that ensures that the computer meets the requirements to boot up properly. In case the computer fails to pass the POST then there will be a combination of beep sounds that indicate a malfunctioning within the computer.
The BIOS usually has 4 main functions:
POST - Before the process of loading the Operating System is initialized, POST tests the computer hardware to see if they are properly functioning.
Bootstrap Loader - It is a process of locating the operating system. In case a active Operating system is found BIOS will pass the control to it.
BIOS - Software / Drivers which interfaces between the operating system and the hardware.
CMOS Setup - It is a configuration program that allows configuring of hardware settings such as computer passwords, time, and date.
