A mod is a modification to Supreme Commander. They change and enhance gameplay by adding units, changing unit abilities, or adding or changing game mechanics. Some mods are called total conversions, which change nearly everything about the game.
Users install mods in the appropriate place in their Supreme Commander file (D:\Supreme Commander\gamedata For example) installation, and then enable them in the Extras menu in Supreme Commander. All players in a multiplayer game must be using the same mods. Some mods (eg UI Mods) can be used by an single player in an mulitplayer game. Those Mods do not change anything on the units.
There are many sites that offer mods for downloads, along with information and reviews. The following are just a few of those sites: