Mods are basically clip-on content that can be added to games. They usually add more weapons, vehicles, and items and often add their own made up lore into the Aground backstory. These clip-ons can be found on mod.io and many other sites.
It is free to make mods for aground and put them on mod.io If you want to try to mod but don’t know how, then join the UAMT discord for help.