Continuing on from what Falling Blue has said,a mod is basically an update of a game updated for visual or bug repairing reasons,to return the game to its original settings you need to completly uninstall the game and delete the mod file - which should be located in a folder/file external to the main games folder.When you reinstall the game it should return to its original settings. The orginal game will be unaffected regardless of the number of mods you install onto it.
Hope that is of some help 🙂