The Log Shipping - Secondary Latency alarm is raised when the amount of time that elapsed between the log backup being created on the primary database and restored on the secondary database exceeds a threshold.
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When the alarm is raised it may indicate a problem in getting data from the primary database to the secondary database. Looking at the history of the copy or restore job may help identify the problem. To do this, go to the SQL Server | High Availability drilldown | Secondaries page and select an item in the grid to view the history.

You can configure this alarm to ignore certain values. You can configure different alarm severities for specific secondary databases. From the Spotlight Client see Configure | Alarms.