Extensions are installable packages which give CiviCRM new functionality, and this directory provides a centralized list of extensions which the CiviCRM community has created.
Further resources:
This listing displays modules specifically built to integrate with features from Backdrop. You can also browse directories of plugins or modules that are made specifically for Drupal, Joomla and WordPress. The recommended way to add functionality to CiviCRM is using CiviCRM extension.