An unacceptable number of logs on the Standby are waiting to be processed by the Apply Server. Is there a problem with the Apply Server or the processes following the Apply Server?
The thresholds of this alarm should align with your business requirements for Recovery Time Objective (RTO). Should the primary go down, these logs need to be processed before the Standby can replace the Primary which will take time. For more information on Recovery Time Objective (RTO), refer to the Oracle Database Help Center.