divi sūrya-sahasrasya
bhaved yugapad utthitā
yadi bhāḥ sadṛśī sā syād
bhāsas tasya mahātmanaḥ
divi - in the sky; sūrya - of suns; sahasrasya - of many thousands; bhavet - there were; yugapat - simultaneously; utthitā - present; yadi - if; bhāḥ - light; sadṛśī - like that; sā - that; syāt - might be; bhāsaḥ - effulgence; tasya - of Him; mahā-ātmanaḥ - the great Lord.
What Arjuna saw was indescribable, yet Sañjaya is trying to give a mental picture of that great revelation to Dhṛtarāṣṭra. Neither Sañjaya nor Dhṛtarāṣṭra was present, but Sañjaya, by the grace of Vyāsa, could see whatever happened. Thus he now compares the situation, as far as it can be understood, to an imaginable phenomenon (i.e., thousands of suns).