XNB

Stardew Valley stores data in .xnb files. XNB files are like zip files, in that they contain more files inside of them. Old mods edited xnb files directly and were installed by replacing the base game files, but this is no longer recommended thanks to the introduction of SMAPI, which allows users to add and remove mods easily and without risk of breaking their game. XNB mods can only edit existing content and cannot add new content, such as maps and NPCs, like Category:Content Patcher mods can. For systems without SMAPI access, such as consoles, XNB mods are the only way to mod the game (but are still not recommended as making a console game accessible for modding usually involves jailbreaking the console, which is usually against the TOS.) XNB mods may also be utilized in non-English speaking communities where Content Patcher instructions and documentation have not been translated and made accessible.

From the Discord server:

XNB mods often break the game and are not recommended. See https://stardewvalleywiki.com/Modding:Using_XNB_mods for more info (and a list of Content Patcher alternatives), and https://stardewvalleywiki.com/Modding:Player_Guide/Troubleshooting#Reset_your_content_files to fix problems caused by XNB mods.

For mod creators, see https://stardewvalleywiki.com/Modding:Editing_XNB_files for help unpacking & editing them (including for use with Content Patcher).