A mod is a change made to an original game by someone other than the creator of the game.
When a player who is playing a Pokemon Game would like to change something about it, they’ll trade their game disk with other players or download illicit software to modify files on their own copy. As this kind of work can be difficult and also illegal (depending on which region you live in) some people choose instead just to use patches which are mods applied by the developers after release into order to offer fixes for problems that players have found with the base version.