This monitor rolls up the data synchronization state of all availability replicas in the availability group and check whether any availability replica’s synchronization is not operational. The monitor is in unhealthy state if any of availability replica’s data synchronization state is NOT SYNCHRONIZING. The monitor is healthy state is none of availability replica’s data synchronization state is NOT SYNCHRONIZING.
In this availability group, at least one secondary replica has a NOT SYNCHRONIZING synchronization state and is not receiving data from the primary replica.
Find the availability replica with NOT SYNCHRONIZING state using availability replica monitor state and resolve the issue at the availability replica.