what will happen if we directly call run method?

I have a class "TestRunnable" which overrides run method by implementing Runnable. Running overridden run method, as follow :

TestRunnable nr = new TestRunnable();
Thread t = new Thread(nr);
t.setName("Fred");
t.start();

• What if i directly call t.run();
• What happen if we don't call t.start(); ?