Some mods let you drop files into their folder to extend the game. That includes Advanced Location Loader (add locations), Json Assets (add crops and items), Seasonal Immersion (make sprites seasonal), and several others.
That's very convenient for modders, who don't need to write their own SMAPI mods. But these files aren't installed the same way as normal mods, and each mod has its own structure and rules for how you add the files. That means players are often confused about how to install them.
SMAPI 2.5 will add content packs to support this use case, designed with input from the authors of those mods. Content packs are installed by dropping them directly into your Mods folder (just like a SMAPI mod), and they automatically benefit from SMAPI's update alerts, compatibility checks, logs for troubleshooting, etc.
Content packs are listed separately in the SMAPI console:
XNB mods have a lot of problems: no compatibility or update checks, confusing to install, hard to uninstall, very hard to troubleshoot, often incompatible with each other, and frequently broken by game updates.
Content Patcher is a new mod that aims to make XNB mods obsolete.
For players, it uses content packs to make installation way easier — instead of manually replacing various game files, just drop packs into your Mods folder. This also adds compatibility and update checks, something XNB mods have never had. If you run into problems, the log shows which content packs edited which files, and you can just remove their folder under Mods to uninstall them.
For modders, this combines the main benefit of XNB mods (no programming needed) and SMAPI (flexible content API). The JSON format is simple and well-documented, and lets you edit part of a file without replacing the whole thing. That means content packs will rarely conflict (unlike XNB mods), and are easy to adjust when they do. See the docs for creating a Content Patcher content pack.
The end of XNB mods?
Stardew Valley mods have always been split into two types: XNB mods (which replace game files) and SMAPI mods (which use SMAPI). This causes a lot of player confusion, in addition to the XNB disadvantages I mentioned above.
Despite those disadvantages, modders keep creating them because they don't require programming. That's been the main barrier to creating more user-friendly SMAPI mods, a problem addressed by content packs and Content Patcher.
After SMAPI 2.5 and Content Patcher 1.0 go live, I'll be helping authors migrate existing XNB mods to content packs. One day soon you might be able to drop any mod into your Mods folder, no matter what kind it is. :)