A Thread Pool is an abstraction that you can give a unit of work to, and the work will be executed by one of possibly several threads in the pool. One motivation for using thread pools is the overhead of creating and destroying threads. Creating a pool of reusable worker threads then repeatedly re-using threads from the pool can have huge performace benefits for a long-running application like a service.
concurrent-ruby also offers some higher level abstractions than thread pools. For many problems, you will be better served by using one of these -- if you are thinking of using a thread pool, we especially recommend you look at and understand Futures before deciding to use thread pools directly instead. Futures are implemented using thread pools, but offer a higher level abstraction.
But there are some problems for which directly using a thread pool is an appropriate solution. Or, you may wish to make your own thread pool to run Futures on, to be separate or have different characterist