The BIOS is special software that interfaces the major hardware components of your computer with the operating system. It is usually stored on a Flash memory chip on the motherboard, but sometimes the chip is another type of ROM.
When you turn on your computer, the BIOS does several things. This is its usual sequence:
Check the CMOS Setup for custom settings
Load the interrupt handlers and device drivers
Initialize registers and power management
Perform the power-on self-test (POST)
Display system settings
Determine which devices are bootable
Initiate the bootstrap sequence