In plain words
The embodied one in every body is forever invulnerable, Bharata. Therefore you should not grieve for any being.
Word for word
This embodied [one] in the body of everyone is always unslayable, O Bhārata; therefore you ought not to grieve for all beings.
The commentators
Bhagavadgītābhāṣyam
अथेदानीं प्रकरणार्थमुपसंहरन्ब्रूते —
देही शरीरी नित्यं सर्वदा सर्वावस्थासु अवध्यः निरवयवत्वान्नित्यत्वाच्च तत्र अवध्योऽयं देहे शरीरे सर्वस्य सर्वगतत्वात्स्थावरादिषु स्थितोऽपि सर्वस्य प्राणिजातस्य देहे वध्यमानेऽपि अयं देही न वध्यः यस्मात् , तस्मात् भीष्मादीनि सर्वाणि भूतानि उद्दिश्य न त्वं शोचितुमर्हसि ॥ ३० ॥
In plain words
Now, gathering up the purport of this whole section, he says —
The one who inhabits the body, who owns it in some sense without ever being it, cannot be harmed -- not ever, not in any state at all -- simply because it has no parts to be broken and no ending to be brought about. It's present, untouchable, in every single body, everywhere, in every kind of living thing down to the plants that never move. So even while a body is being cut down in this war, the one who dwells in it is not being touched at all. That's the whole reason you have no cause to grieve over Bhishma, or any of them.
Close to the Sanskrit
Now, at this point, summing up the meaning of the section, he says —
The embodied one, the possessor of a body, is eternally, at all times, in all states, inviolable -- because of being without parts and because of being eternal. There, this [Self] is inviolable in the body, in [any] body, of everyone -- because it is all-pervading, abiding even among the immovable [beings, such as plants] and the rest, of every class of living being; even while the body is being slain, this embodied one is not slain -- because [of the reasons just given]. Therefore, regarding all these beings, Bhishma and the rest, you should not grieve.