अथ त्रयो वाव लोका मनुष्यलोकःपितृलोको देवलोक इति सोऽयं मनुष्य-लोकः पुत्रेणैव जय्यो नान्येन कर्मणा
In plain words
Now, there are indeed three worlds: the human world, the realm of the ancestors, and the realm of the gods. Of these, the human world is to be won only through a son, and by no other action.
Close to the Sanskrit
Now, there are indeed three worlds: the human world, the world of the fathers, and the world of the gods. This human world is to be won only through a son, and by no other action.