The DBSS - Worker Thread alarm is part of the SQL Server Connections Summary collection.
The alarm fires with the message
"SQLServer instance -value: The percentage of Worker Threads used (-value%) is greater than the defined threshold -value%."
In general, the calculation is based on a determination of the active workers in the system (the difference in the count of the active_workers from sys.dm_os_schedulers that are visible online) from the count of system workers (from sys.dm_exec_requests and sys.dm_exec_sessions).
Divided by the MaxWorkerThreads from sys.configurations.
Refer to the following external web pages for more details on some of the metrics used in this calculation
Refer to KB article 272130 regarding requests for the specific SQL statement used in the DBSS - Worker Thread collection.