In staging mode, the server is active for import and synchronization, but it does not run any exports. A server in staging mode is not running password sync or password writeback, even if you selected these features during installation. When you disable staging mode, the server starts exporting, enables password sync, and enables password writeback.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-staging-server
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-operations