An unacceptable number of logs on the Primary are waiting to be shipped through the Archiver to the Standby. he logs waiting on the Primary will be lost should the Primary go down. The thresholds of this alarm should align with your business requirements for Recovery Point Objective (RPO).
For more information on Recovery Point Objective (RPO), refer to the Oracle Database Help Center.
Note: The cause of the problem could be a bottleneck further on in the process. If the Network is slow for example then the Archiver may be prevented from running at maximum capacity which could result in too many logs waiting on the Primary.