Usually SQL Server opens system thread for each query request, but if amount of threads exceeds specified max worker threads value, SQL Server pools the worker threads. When all worker threads are active with long running queries, SQL Server may appear unresponsive until a worker thread completes and becomes available. Though not a defect, this can sometimes be undesirable. The monitor analyzes amount of free threads and notifies if the amount is low.
An increase in the amount of work causing an increase in utilized threads, this could indicate that SQL Server is working under significant load or an excessive number of queries running in parallel.
Adjusting max worker threads is an advanced option and should be changed only by an experienced database administrator or certified SQL Server technician.
Thread pooling helps optimize performance when large numbers of clients are connected to the server. Usually, a separate operating system thread is created for each query request. However, with hundreds of connections to the server, using one thread per query request can consume large amounts of system resources. The max worker threads option enables SQL Server to create a pool of worker threads to service a larger number of query requests, which improves performance.
Max worker threads Option