'This (body) indeed withers and dies when the living Self has left it; the living Self dies not. That which is that subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.

In plain words: The body dries up and dies when the living Self leaves it, but the living Self itself never dies. This tiny hidden essence is the true self of everything, and the father tells Svetaketu, "You are that." The son asks to learn more, and the father agrees.