Actually, the explanation of this mantra and of practically all the mantras of the Vedic hymns is summarized in the Vedānta-sūtra and properly explained in Śrīmad-Bhāgavatam. Śrīmad-Bhāgavatam is the mature fruit of the Vedic tree of wisdom. In Śrīmad-Bhāgavatam this particular mantra is explained in the questions and answers between Mahārāja Parīkṣit and Śukadeva Gosvāmī at the very beginning of their meeting. Hearing and chanting of the science of God is the basic principle of devotional life. The complete Bhāgavatam was heard by Mahārāja Parīkṣit and chanted by Śukadeva Gosvāmī. Mahārāja Parīkṣit inquired from Śukadeva because Śukadeva was a greater spiritual master than any great yogī or transcendentalist of his time.
Mahārāja Parīkṣit's main question was: "What is the duty of every man, specifically at the time of death?" Śukadeva Gosvāmī answered:
- tasmād bhārata sarvātmā
- bhagavān īśvaro hariḥ
- śrotavyaḥ kīrtitavyaś ca
- smartavyaś cecchatābhayam
"Everyone who desires to be free from all anxieties should always hear about, glorify and remember the Personality of Godhead, who is the supreme director of everything, the extinguisher of all difficulties, and the Supersoul of all living entities." (SB 2.1.5).
So-called human society is generally engaged at night in sleeping and having sex and during the daytime in earning as much money as possible or else in shopping for family maintenance. People have very little time to talk about the Personality of Godhead or to inquire about Him. They have dismissed God's existence in so many ways, primarily by declaring Him to be impersonal, that is, without sense perception. But in the Vedic literature-whether the Upaniṣads, Vedānta-sūtra, Bhagavad-gītā or Śrīmad-Bhāgavatam—it is declared that the Lord is a sentient being and is supreme over all other living entities. His glorious activities are identical with Himself. One should therefore not indulge in hearing and speaking of the rubbish activities of worldly politicians and so-called big men in society but should mold his life in such a way that he can engage in godly activities without wasting a second. Śrī Īśopaniṣad directs us toward such godly activities.
Unless one is accustomed to devotional practice, what will he remember at the time of death, when the body is dislocated, and how can he pray to the almighty Lord to remember his sacrifices? Sacrifice means denying the interest of the senses. One has to learn this art by employing the senses in the service of the Lord during one's lifetime. One can utilize the results of such practice at the time of death.