Bhagavad Gita Verse 21, Chapter 2

vedaavinaashinam nityam ya enamajamavyayam |
katham sa purushaha paartha kam ghaatayati hanti kam || 21 ||
 
Whosoever knows this (eternal essence) to be imperishable, eternal, birthless and changeless, how can that individual, O Paartha, slay or cause anyone to be slain?
 
veda : knows
avinaashinam : imperishable
nityam : eternal
yaha : whosoever
enam : this
ajam : birthless
avyayam: changeless
katham : how
saha : that
purushaha : individual
paartha : O Paartha
kam : who
ghaatayati  : cause to be slain
hanti : kill
kam : who
 
Imagine that you are operating a laptop that is connected to a printer. You open a document on the laptop and click the print button. The laptop sends a signal to the printer, and the document gets printed.
 
Now imagine that the laptop and the printer have egos and can think for themselves. The laptop will say "I initiated the printing action" whereas the printer will say "I was the receiver of the printing action".
 
But in reality, an electric current went from the laptop to the printer, which then caused the printing to happen. So, an engineer would never say "the laptop caused the printing to happen". It was all a play of electricity.
 
So here, what Shri Krishna is trying to say is that "this", the eternal essence does not act and nor does it get impacted by any action, just like electricity does not really act. It empowers material objects to act without actually acting. Therefore a wise person will never think that the eternal essence can slay or kill or act at all. It is like the sun.
 
The refrain is clear: "You are the eternal essence - birthless, changeless, eternal and imperishable. The eternal essence does not slay, nor does it get slain." It is repeated throughout the Gita because it will take a significant effort on our part to truly understand and internalize this message.
 
This shloka forms the seed of chapter thirteen of the Gita on the topic of knowledge of the eternal essence, or "tattva jnyaana".

Bhagavad Gita Verse 17, Chapter 2

avinaashi tu tadviddhi yena sarvamidam tatam |
vinaashamavyayasyaasya na kashchitkartumarhati || 17 ||
 
But know that (eternal essence) to be indestructible by which all this is pervaded. Nothing ever can destroy that, the imperishable.
 
avinaashi : indestructible
tu : but
tat : that
viddhi : know
yena : which
sarvam : everything
idam : this
tatam : pervades
vinaasham : destruction
avyayasya :  of the imperishable
asya : of this
na : not
kashchit : any
kartum : do
arhati : is able
 
The theme of the ongoing shlokas has been understanding the nature of the eternal essence. This verse gives us two qualities that the eternal essence possesses. Note that the sanskrit word "tat" meaning "that" refers to the eternal essence in this shloka.
 
Firstly, the eternal essence is imperishable. It cannot be destroyed, nor is it created. In the bangle example from the prior verse, the pawn shop owner only cared about the gold content of the bangle and not its shape and form. He could melt that bangle into another ornament, melt it again and make it into another ornament, and so on. In doing so, each subsequent ornament was "created" and "destroyed", but the gold essence was imperishable and indestructible. Similarly, the eternal essence spoken of in these verses is imperishable.
 
Secondly, the eternal essence pervades "all this", which means the eternal essence pervades the entire universe. If we immerse a piece of cloth into water, water pervades each and every fibre of the cloth. There is no part of the cloth that isn't dry. In the same way, from a rock, to plants, to animals, and to humans, the eternal essence pervades everything that is part of the universe.