Availability replica is not joined
This policy checks the join state of the availability replica. The policy is in an unhealthy state when the availability replica is added to the availability group, but is not joined properly. The policy is otherwise in a healthy state.
For this release of SQL Server 2014, information about possible causes and solutions is located at Availability replica is not joined on the TechNet Wiki.
Use Transact-SQL, PowerShell, or SQL Server Management Studio to join the secondary replica to the availability group. For more information about joining secondary replicas to availability groups, see Joining a Secondary Replica to an Availability Group (SQL Server).