This is not a warning about the lack of quality bedding in your hotel while at a conference. This alert warns you when SQL Server is running low on available worker threads.
This Advisory Condition will check to see how many worker threads are possible in your SQL Server, then deduct the amount used to provide available threads for use.
The number of available threads is then checked to see if it is less than or equal to 5% of the total threads. If this is returned as true then the alert will fire.
If you wish to change the percentage then this can be done by changing the query being compared against.
Resolutions
It is possible to change the number of workers available in SQL Server but this is generally not advised (as per Microsoft Documentation - https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/configure-the-max-worker-threads-server-configuration-option?view=sql-server-ver15)
Instead you should see what is using the active threads and tune the statements involved to improve concurrency. Please go to the Top SQL tab to see what queries were running during the timeframe this alert fired.
Further Reading on threads - https://docs.microsoft.com/en-us/sql/relational-databases/thread-and-task-architecture-guide