Cardinal Components is an API for attaching arbitrary data to various game objects.
If you are a developer, be sure to check the repository's readme for an overview of this library's capabilities : https://github.com/OnyxStudios/Cardinal-Components-API
If you are a regular minecraft player, you may need this API to play with your favorite mods. Just download the version that works for you - Cardinal Components API does not have any user-facing feature by itself.