Drupal uses a synchronization folder to hold the configuration YAML files that are to be imported into the current site. This folder is represented by the CONFIG_SYNC_DIRECTORY constant. If you have not defined this in the global $config_directories variable in your site's settings.php, then it will be a randomly named directory in your site's file directory.
When Drupal 8 entered its beta release cycle, this folder was referenced as a staging folder and referenced by the CONFIG_STAGING_DIRECTORY. This is now deprecated; however, the internals of the configuration management system support reading CONFIG_STAGING_DIRECTORY as CONFIG_SYNC_DIRECTORY. This will be removed in Drupal 9.
The synchronization form will use the configuration management discovery service to look for configuration changes that need to be imported from this folder.