There are two types of Wii mods out there. There is a “hard mod” and a “soft mod”.
A hard mod usually involves opening you Wii and buying a “mod chip” a mod chip properly installed will allow you to backup copies of your original DVD games and play burned discs, keeping your originals out of harms way.
A soft mod is a way of reprogramming your Wii’s internal flash that stores the core Wii files. This requires no tools or additional hardware. The soft mod will allow you to run unsigned or “home brew” code on your Wii and perform such tasks as backing up your games to a USB hard drive and loading them without have to use your DVDs.