अन्तवन्त इमे देहा नित्यस्योक्ताः शरीरिणः। अनाशिनोऽप्रमेयस्य तस्माद्युध्यस्व भारत ॥

antavanta ime dehā nityasyoktāḥ śharīriṇaḥ anāśhino ’prameyasya tasmād yudhyasva bhārata

These bodies of the embodied One, who is eternal, indestructible and immeasurable, are known as finite. Therefore fight, O Bhârata.

In plain words: Bodies come to an end, but the soul inside them is eternal, cannot be destroyed, and cannot be measured. So Krishna tells Arjuna to fight.