This list is automatically downloaded from available and compatible native extensions on the CiviCRM server. To find out more about a specific extension, click the arrow on the left side of the extension name. This will reveal more details about this extension including the author, release version, release date and licensing.
CiviCRM will find, use and (via the web interface) manage extensions in the url and path location defined above, the "default" extension location for your install.
But CiviCRM will also find and use extensions in other locations. These locations might be populated by other mechanisms - e.g. a vetted subset of all extensions that are made available by default by a hosting provider. Specifically, CiviCRM will scan three directories and their subdirectories for info.xml files, similar to the mechanism that Drupal uses to automatically detect modules (in fact, using almost identical code). The three directories correspond to three extension 'containers', as defined in the function "getFullContainer" of the file "CRM/Extension/System.php"