Srimad BhagavadGita

With the purpose of explaining to Arjuna that we should not grieve for the soul, Kṛṣṇa described the eternal unchanging nature of the soul in the previous śloka. Knowing that Arjuna would get the doubt that, 'If souls are eternal, how are birth and death possible?', Kṛṣṇa cleared it in this śloka by describing the body's transient and destructible nature with an example. Kṛṣṇa said, “Just as the embodied soul passes through childhood, youth and old age in this body, similarly soon after leaving this body, the process of obtaining another body also occurs. The wise are not deluded about leaving this body and taking up another body.”

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dehinaḥ The jīva is addressed as dehī here because he contains a deha. The jīva passes through the phases of childhood, youth, etc, in this body. However while the jīva is in this body, he cannot transmigrate to another body.

asmin dehe(vartamānasya) dehinaḥ yathā kaumāraṃ yauvanaṃ jarā When the words are rearranged in this manner, accurate meaning can be derived which is as follows- 'As the body undergoes the transformations of childhood, youth and old age when the jīva is present in the body'. When read as dehinaḥ asmin dehe yathā kaumāraṃ yauvanaṃ jarā the meaning derived is that, 'As the embodied jīva in this body has stages like childhood, youth and old age' which is not acceptable, as it is not the eternal changeless jīva that undergoes these stage changes. It is the body which undergoes these changes while the jīva is present in it.

kaumāraṃ yauvanaṃ jarā When the jīva is in a body, the body first has the childhood stage. After it is over, the youth stage dawns, followed by old age. In this way, each stage comes after the previous one is over.

In all these stages, changes take place in the body alone and not the soul. The soul remains unchanged. At the time of each of these stage changes, a wise person does not grieve that 'Oh! I(soul) am getting destroyed'. Similarly, the wise do not grieve even when the soul leaves the present body and takes up another one.

tathā dehāntara prāptiḥ Just as these different stages come in the body of the embodied soul one after the other, the stage of shedding the present body and taking up a new body also happens for the soul.

At each transition one may feel that, 'Oh! The previous stage is gone', but no one will grieve that the soul itself is destroyed. Similar is the case when the present body is lost and new one is taken up by the soul, one can feel that, 'Oh! This body is gone', but not that the soul itself is destroyed.

dhīras tatra na muhyati One who has this knowledge knows that just as the transformations of childhood, youth and old age in the body are inevitable for the embodied soul, so also are the destruction of this body and acquisition of a new body inevitable. Hence, he will not grieve for this transformation too. As a wise person knows that the soul is eternal, there is no chance that he will grieve for it.

Summary:

By explaining that the soul is never destroyed and that the ever changing body has to get destroyed one day or the other, Kṛṣṇa was telling Arjuna that he should not step back from his duty of fighting the war fearing the death of his relatives. Arjuna might say that, 'As the soul is eternal, I do not grieve that it will be destroyed. But fighting the battle will cause me pain due to arrows, etc, that will wound my body. Seeing my dear ones in pain due to similar wounds on their bodies, will also cause pain to me. Why should I endure all this pain? If I back off now itself, I can avoid all this pain.' Kṛṣṇa has answered this in the next two ślokas.