In plain words
The varṇas are mixed. Dharma perishes. So, because Arjuna grieved with Dharma as his aim and God in his sight, this viṣāda turned into yoga. It is no grieving for wealth and gold, goods and vehicles. He goes on grieving only for God's sake and for Dharma's sake. That is why its name is śokāgni. Baḍabāgni is to burn with dreadful diseases and with death. These five kinds of fire belong only to the physical body. This has no connection at all with the Ātma, which is other than it. To these five kinds of fire he remains a witness. To proclaim this principle of the witness to the world, the Upaniṣad took the measures of these pañcāgnis as its ground and taught the people.
In it there is one more inner secret too. Among men there are four kinds of class. The divine, the human, the demonic and the bestial. Of these natures, the divine, the human, the asura and the bestial, the very first is man as divine. Daivaṃ mānuṣarūpeṇa. ‘Brahmaniṣṭha bhavo viṣṇuḥ’: it holds the form of Viṣṇu in the heart. To fill the heart with divinity, to do every karma to please God, to believe that the very principle of the Ātma that is in all is in oneself, to love all beings with compassion and Prema, to believe in this feeling of the one Ātma, and to live one's life in a spirit offered to God: this alone is called man's divinity. The second. Man as human. Satya dharma bhavo martyaḥ: that is, in this mortal world, to keep Satya and Dharma in view, to spread and teach Satya and Dharma, to love people with compassion, kindness and joy, to recognise that man has come only to foster humanness, to understand that man must live as man, to give and do good, to make thought, word and deed one, and, though he is a householder in his life, to carry out the karmas of his duty and go on showing forth a divine life as the human ideal. This is called man as human; and next, man as demon. ‘Surāpānaṃ bhavo dānavaḥ’. To do nothing but drink liquor, harm living beings, torment others for selfish ends and self-interest, serve his own pleasure, and, without compassion, without Prema, without mercy, lead a life of selfishness and self-interest: the Upaniṣad has set this forth as the nature of a demon.