Thread.currentThread()

What this is for instances is Thread.currentThread() for threads. It returns the threads reference variable in the environment the code is called. When you create and start a thread in the main method, you have a Thread reference variable (t in the example)

Thread t = new MyThread();
t.start();

The now running thread doesn't know about that reference variable anymore as you didn't pass it. But you have the possiblity to get it with currentThread() and now you can call methods on that object again, like isInterrupted or sleep.