MongoDB admit that this license is not currently open source in the sense of being approved by the Open Source Initiative, but say:We believe that the SSPL meets the standards for an open source license and are working to have it approved by the OSI.
At the broadest level, AGPL requires you to distribute the source code to the AGPLed work[1] while the SSPL requires you to distribute the source code to everything involved in providing the service.