In plain words
You are Vayu, Yama, Agni, Varuna, the moon, Prajapati, and the great-grandsire. Salutations to You a thousand times; again and yet again, salutations to You.
Word for word
Vāyu, Yama, Agni, Varuṇa, the moon, Prajāpati [are] You, and the great-grandfather; salutation, salutation be to You a thousand times, and again, and yet again, salutation, salutation to You.
The commentators
Bhagavadgītābhāṣyam
किञ्च —
वायुः त्वं यमश्च अग्निः वरुणः अपां पतिः शशाङ्कः चन्द्रमाः प्रजापतिः त्वं कश्यपादिः प्रपितामहश्च पितामहस्यापि पिता प्रपितामहः, ब्रह्मणोऽपि पिता इत्यर्थः । नमो नमः ते तुभ्यम् अस्तु सहस्रकृत्वः । पुनश्च भूयोऽपि नमो नमः ते । बहुशो नमस्कारक्रियाभ्यासावृत्तिगणनं कृत्वसुचा उच्यते । ‘पुनश्च’ ‘भूयोऽपि’ इति श्रद्धाभक्त्यतिशयात् अपरितोषम् आत्मनः दर्शयति ॥ ३९ ॥
In plain words
And there is more —
You are the wind, you are Yama, you are fire, you are Varuṇa who governs the waters, you are the moon riding the sky, you are Prajāpati, lord of every creature. You are Kaśyapa and the first fathers of all — the great-grandfather, meaning: father even of the grandfather, meaning: father even of Brahmā himself. Salutations to you, salutations, a thousand times over. And again — and still again — salutations, salutations to you. Śaṅkara points out that this doubled, thousandfold bowing is exactly what the grammar of 'so many times' exists to say. And in saying 'again' and 'yet again,' Arjuna shows us his own heart laid bare: his overflowing śraddhā and bhakti leave him still unsatisfied, however many times he bows.
Close to the Sanskrit
Moreover —
You are Vāyu, and Yama, Agni, Varuṇa the lord of the waters, Śaśāṅka — [that is,] the moon — you are Prajāpati, [and] Kaśyapa and the rest; [the verse's word] 'great-grandfather' means: even the father of the grandfather is the great-grandfather — meaning, even the father of Brahmā is meant. Salutation, salutation to you, to you, let it be, a thousand times. And again, yet again, salutation, salutation to you. The reckoning of the repeated performance of the act of salutation, done in many ways, is expressed by [the suffix] kṛtvasuc. By [the words] 'and again' [and] 'yet again' he shows, from the excess of his śraddhā and bhakti, his own lack of full contentment.