Mods are modifications for video games. Mods are fun because you can customize the game.
Modding was first popularised on Doom. It is still relevant today and some games are even meant to be modified. For example: Gmod modifying has advanced a long way since Doom with Steam the online video game distributor having their own platform for mods called the Steam workshop.
Modding is more popular than you might think. Many popular games can be modified and are known for their massive number of mods. Some games started as mods, such as
Counter-Strike created by Minh Le and Jess Cliffe based on Half-life, DAYZ created by Dean Hall which is based on ARMA 2, Garry’s mod created by Garry Newman which is based on Half-life 2, PUBG by Brendan Greene based on ARMA 2, The Stanley Parable created by Davey Wreden based on Half-Life 2, Team Fortress by Robin Walker and John Cook based on Quake.
Mods are important because old games can see resurgence in sales with popular mods.