Extensions are installable packages which give CiviCRM new functionality, and this directory provides a centralized list of extensions which the CiviCRM community has created. This listing displays CiviCRM extensions that work on all Content Management Systems (CMS).
For example if it is installed on a CiviCRM system it will minify crm-menubar.css. If Stripe is installed it will minify stripe css/js.
Requires - CiviCRM 5.x, php 5.6
Credit : the Wikimedia Foundation