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 break a mirror and the glass is ruined. The light that was in it is not damaged, because it was never a property of the glass. Your hand had power over one of those things and never had any over the other.
Going deeper
The word is tatam — stretched through, pervading, the way a thread runs through cloth.
This is the first hint of something the Gita will expand enormously in Chapters 7 to 11: what is being talked about is not a small private soul sealed inside a body. It is continuous, and the same thing everywhere.
That matters for Arjuna's question, though the full force arrives later. 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.
And notice the shape of the consolation, because it is not a soft one. Krishna is not saying that Bhishma will be fine, or that they will meet again, or that the loss is small. He is saying that the thing Arjuna is frightened of doing is not within anyone's power to do.
That is colder than comfort and more useful. It does not ask Arjuna to feel better about the deaths. It removes the specific horror — being the one who ends someone — that has him on the floor of the chariot.