In plain words
I accept as true all that You tell me, Keshava. For neither the gods nor the danavas know Your origin, Bhagavan.
Word for word
All this I hold true, which You speak to me, O Keśava; for neither do the gods, O Bhagavān, know Your coming forth, nor the Dānavas.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
सर्वमेतत् यथोक्तम् ऋषिभिः त्वया च एतत् ऋतं सत्यमेव मन्ये, यत् मां प्रति वदसि भाषसे हे केशव । न हि ते तव भगवन् व्यक्तिं प्रभवं विदुः न देवाः, न दानवाः ॥ १४ ॥
In plain words
All of it — everything the seers have declared and everything you say yourself — I take as ṛta, as simply true: whatever you are telling me, Keśava. Because no one knows how you come to appear, Bhagavān, no one knows your origin: not the gods, not the dānavas.
Close to the Sanskrit
All this, as stated by the seers and by you — this I consider ṛta, that is, simply true — whatever you speak, that is, utter, toward me, O Keśava. For indeed your — of you, O Bhagavān — manifestation, that is, origin, they do not know: not the gods, not the dānavas.