how to use queueuserworkitem in threading in MCTS 70-536

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.NET3.5

C#

.NET

C#4.0

I have created a program which is given in MCTS 70-536 exam....but not able to understand its coding...So i need support to learn this...

class Program
    {
        static void Main(string[] args)
        {
            ThreadPool.QueueUserWorkItem(ThreadProc);
            Console.WriteLine("Main thread does some work, then sleeps.");
            Thread.Sleep(5000);
            Console.WriteLine("Main thread exists.");
            Console.ReadLine();
        }
        static void ThreadProc(object a)
        {
            Console.WriteLine("Hello from the thread pool.");
        }
    }

Thanks in advance