Mods
A mod is any alteration to the base game, through modding.
Mods are made up of a single .pack file, which contains all the changes for that mod, and a single .png file of the same name - which is just used as the thumbnail within Steam and the Total War Launcher.
Finding Mods
Most mods that you will find can be discovered on the Steam Workshop page, for each Total War game.
Point of assistance here - you'll have a better time either Googling for the mods you're looking for (ie., "total war warhammer 2 greenskin units mods"), or searching for a collection on the Workshop. Steam Workshop is... a subpar searching service. Ironically.
Installation
When you find a mod you like in the Steam Workshop, you simply press the "Subscribe" button - making sure to download all required items for that mod! - and let Steam download it in the background. Next time you load up the game, you will see it available in the launcher's mod manager. Simply tick the mod in the manager, and it will be enabled next time you press play!
Important note here: Don't faff around with load order. Mods are sorted alphanumerically by default, and modders are aware of the priorities needed between mods most of the time, when it comes to submods or similar.
Troubleshooting
If a mod doesn't appear to work or update, you can try to unsubscribe and re-subscribe, though in some instances this may not be enough and you may need to Clear the Steam Download Cache, see this article
See also: Troubleshooting