Modding, in the videogame world, is adding, altering, or purging content on a videogame, especially on the pc. Mods can be quests, items, houses for the player, towns, shops, factions, or altering technical stuff(scripts, textures, meshes). There are also
total conversion mods, which are mods to an insane degree
A notable
moddable game is Oblivion, but beware, buying this game means you will
sell your soul(assuming gamers have souls)