Hi sashankrm,
Modding BIOS means that you go into your motherboard's Basic Input/ Output firmware and make changes. The changes that you make in BIOS can range from how much voltage you choose to give your components, to what order your devices run during booting up.
The average user doesn't need to touch the BIOS settings, as there are lots of things that can go wrong with your PC if you make incorrect changes.
If you are curious about what your BIOS looks like, you can press a certain key (usually f12, f8, or del) right as your computer boots up to go into the BIOS. It doesn't hurt to look around, just make sure you don't change any settings by accident!