A "mod" is essentially just a modification. If you saw the topic I made, (Future: VV2 mods?) you'll notice I wanted a mod that changed the colors of the mushrooms, as brown mushrooms are really hard for me to see. Someone who knew how to do it, made one for me, so now I have bright purple mushrooms in my game. I made a backup copy of the original file that I was wanting to "mod" (modify). It's safe in another place, so if I want to put everything back the way it was, I can just take my backup copy, and overwrite the "modded" file with it.