In plain words
The moon is the lamp at nightfall, the sun the lamp at daybreak; Dharma is the lamp of the three worlds, and a good son the lamp of his family.
The one who gives light at night is the moon. The one who gives light by day is the sun. What brings light and radiance to the three worlds is Dharma. A good son makes his family shine.
You know the difference between the new-moon night and the full moon. The new-moon night is a dreadful night, pitch dark. Darkness is frightening. And not fear alone. It breeds many evil thoughts. It leads into evil deeds. Darkness is a great help to thieves and the wicked. So only the wicked and evil-doers long for this darkness; the good-hearted do not want it at all. Those of good resolve, good deeds and good conduct long only for light.
At the sight of the full moon's light, cool and peaceful feelings and good resolves grow in the human heart. Even small children are overjoyed to see the moonlight. Poets, when they see the moonlight, weave into poetry the good feelings that rise in them. On the full-moon day the human heart too is full. We must reflect well on the close bond between man's mind and the moon. ‘The moon was born of the mind, the sun was born of the eye.’ The mind is the very reflection of the moon. So in the moonlight man's mind too grows cool, with good feelings and good resolves. So the full moon drives out the darkness, dispels fear, and fosters peace, security, joy and good resolves.
Next, it has been said, ‘Prabhāte dīpako raviḥ.’ By day the sun gives light to all. Without the sun there would be no creation at all. There would be no crops. No trees. No air. No food. Living beings could not survive. Crops ripen only because of the sun's light. Rivers flow. The world grows in joy, well-being and plenty. Through the very power of the sun, man's heart too is made pure. The hydrogen and helium in the sun help this world in many ways. So the sun is the visible God of this earth.
‘Trailokye dīpako dharmaḥ.’ Dharma gives light to these three worlds. What is Dharma? ‘Dhṛyate iti dharmaḥ’ means that which holds. That which upholds is dharma. Dharma holds everything together. Without Dharma the world itself could not stand. Dharma is like the very life-breath of every action. Without Dharma, which is life itself, there is no living at all. ‘Dhārayatīti dharmaḥ’: Dharma is that which bears. Each thing has a dharma of its own. For example, the power to burn is the dharma of fire. If it loses its power to burn, it is no longer fire. It becomes charcoal. The dharma of ice is its very coldness. If it loses its coldness, it is no longer ice. In the same way, what is the Dharma of man? Heart, word and deed: human Dharma is nothing but the unity, holiness, purity and spotlessness of this triad. That is why it has been said, the proper study of mankind is man.
All the actions a man does must be holy. They must satisfy his conscience. All the words he speaks must be pure. That is why, ‘Jihve rasajñe madhura priye satyaṃ hitaṃ tvāṃ paramaṃ vadāmi’: ‘O holy tongue! Tongue that knows taste! Tongue that knows no selfishness! How great you are!’ So the devotee Jayadeva described it. Today we stain that pure tongue with untruth. We must speak only the truth. And how must we speak even that truth? ‘Anudvegakaraṃ vākyaṃ satyaṃ priyaṃ hitaṃ ca yat.’ The words we speak must not cause agitation. We must not speak hurtful words on the plea that they are true. It must be pleasing. It must do good. We must not talk too much. We must speak gently. That very thing has been called Satya. ‘Trikālabādhyaṃ satyam.’ Satya is that which exists in all three times.
Next, there is something called nijam. In this world people usually take Satya and nijam to be one and the same. But Satya is one thing, and nijam another. Nijam is what is seen only for the moment. Satya is what is seen always. For example, you may be wearing a shawl now. Tomorrow you may not wear it. So that you are wearing a shawl is nijam, but it is not Satya. Whatever has no change or alteration in all three times, whatever neither moves nor stirs, that is Satya. What keeps changing from time to time is not Satya. In Vedānta this Satya has been called ‘Ṛtam’. When it is joined with these two, Jñāna and Viveka, it takes the form called ‘Ṛtam’.
What is our Dharma in our humanness? We must speak Satya. We must practise Dharma. That is man's foremost value.