This is a simple modding API used mainly for compatibility between mods. It does little on its own, but is required for several mods to run.
Navigate to your Hollow Knight Game Install folder.
Steam: C:\Program Files (x86)\Steam\steamapps\common\Hollow Knight\
GoG:
Copy everything in this zip file into the game's folder. When prompted to overwrite files, say yes.
You may also use Scarab.
Steam: Right Click on Hollow Knight -> Properties -> Local Files -> Verify Integrity of Game Files GoG: Options -> Manage Installation -> Verify / Repair