DuckDB's extension mechanism allows for easy extending of DuckDB functionality. Because ducks are not averse to a little dog food once in a while, a significant part of DuckDB's core functionality is provided through this extension mechanism. These extension live in the core repository. This repository is the default extension repository meaning that extensions can be loaded either by specifically specifying the core repository:

INSTALL json FROM core;
LOAD json

or by simply omitting the repository altogether:

INSTALL json;
LOAD json

For an extensive list and documentation of all core DuckDB extensions check out the main DuckDB docs.