'Believe it, my son. That which is the subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.

In plain words: The father says, "Believe it, my son." That hidden essence is the real self of everything, the True, and Svetaketu is that too. The son asks to learn more, and the father agrees.