What is BIOS when building a gaming pc from scratch

[Nibslip]

I am new to PC building and when I watch pc builds they are setting up the BIOS. What is that and once I build my pc will I have to do the same? And if I have to then how would I set it up

 

[RARRAF]

each motherboard comes with its own bios which tells the PC how to work and startup devices that are plugged into it so once you have your hard drives cd drives and such plugged in you will have to enter the bios to tell the PC what you want to boot off of and can be used to overclock