What are BIOS mods and what are they intended for?

Manywhere on the internet, I have seen the term BIOS MODS and it sounds to me that people are altering the BIOS image and/or customizing than the original manufacturer BIOS instruction set.

I want to know, exactly what this BIOS MODS are and if I am right about altering the original BIOS instruction set, what are they intended for?

Thanks in advance.