Additional modules supplied by the Postgres project but not installed by default.

Postgres supplies additional modules not installed in the standard distribution.
The complete list in the current manual.

Before you can use CREATE EXTENSION to load an extension into a database, the extension's supporting files must be installed. There are additional software packages for all common OS available, pre-installed on most DB hosting services, but not all extensions are available on some.

This tag is about issues revolving around handling extensions, not discussing a particular one in depth.