Code:
new ThreadStart (Method1)
Creates a delegate of type ThreadStart. When the delegate is invoked (by calling its 'Invoke' method), it will call Method1.
Code:
new Thread (new ThreadStart (Method1))
Creates a new thread. When the thread is executed (by calling its 'Start' method), it will invoke the ThreadStart delegate.
Code:
Thread firstThread = new Thread (new ThreadStart (Method1));
Assigns a reference to the newly created thread to the firstThread variable.
In a nutshell, this code creates a new thread on which to execute Method1.