From VNDev Wiki

A mod is a modification or addition to a game's code, created by users not officially affiliated with the original game. Mods for visual novels might add a variety of features, such as additional stories, new characters, improvements or adjustments to content from the original game, or even performance improvements.

Some visual novels, such as Doki Doki Literature Club [VNDB], are particularly popular for mod developers. Communities may form around modding particular games, where mod developers share knowledge and advice.