अविनाशि तु तद्विद्धि येन सर्वमिदं ततम्।विनाशमव्ययस्यास्य न कश्चित् कर्तुमर्हति॥
avināśi tu tadviddhi yena sarvam idaṃ tatamvināśam avyayasyāsya na kaścit kartum arhati
Word by word
The verse says
Know that what spreads through all of this cannot be destroyed. Nobody is able to bring an end to what does not decay.
In plain words
One thing pervades everything. That thing cannot be destroyed.
And then the flat statement that answers Arjuna's whole fear: nobody is able. Not should not. Not ought not. Cannot.
Arjuna has been agonising over a power he does not have.
Where you've felt this · in family life
You drop the mirror and the glass is finished. The light that was in it is not damaged, because the light was never a property of the glass. You had power over one of those things and never had any over the other.
Going deeper
What Krishna is describing is not a small private soul sealed inside each body. It is continuous - tatam, stretched through everything, the way one thread runs through a whole cloth.
That has a consequence for Arjuna's question, though the full force of it only arrives in later chapters. If what is real in Bhishma is the same as what is real in Arjuna, then killing him is not the transaction Arjuna thinks it is.
Notice the shape of the consolation, though, because it is not a soft one. Krishna does not say Bhishma will be fine, or that they will meet again, or that the loss is small. He says the thing Arjuna is terrified of doing is not within anyone's power to do.
That is colder than comfort and a great deal more useful. It does not ask Arjuna to feel better about the deaths. It removes one specific horror - being the person who ends someone - and that is the horror that put him on the floor.