A mod* is a modification that can be made to the image files of the game to display something different. Note that programming is not affected by mods, and mods are simply a visual change, not a programming alteration. Since access to the game files is needed to alter the images, mods can only be used on a Windows/Mac version of the game, as LadyCFII stated above.

*not to be confused with Mod, which is a Moderator on these forums