The motherboard manufacturer writes the BIOS code, it is very specific to the individual board type. Even a different revision of motherboard will have it's own code.
BIOS = Basic Input Output System
It is the code that is built into the motherboard to allow the different ICs to be initialized and to let you do things like set the time, which peripherals are enabled and sometimes the system timings. It is the code that for example, lets you use the keyboard and see messages on the screen before the OS has loaded. It sits in the background and does the most basic tasks when the OS has taken charge. Unless you are VERY familiar with the operation of the ICs on the motherboard and have a good understanding of machine code instructions, you should leave it well alone!
Brian.