This error means your connection to the matchmaking service timed out or that the connection between you and another player (or several players) could not be established.

If the error persists, you may find the information in the following FAQs useful:
If you still encounter this error message after completing the connectivity troubleshooting, do not hesitate to contact us. 

If you are looking for more information on error codes in Rainbow Six Siege, please check our list of error codes.