In plain words
Know Me, Partha, as the eternal seed of all beings. I am the buddhi of those endowed with buddhi and the boldness of the bold.
Word for word
Know Me, O Pārtha, [as] the eternal seed of all beings; I am the buddhi of those endowed with buddhi, I the boldness of the bold.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
बीजं प्ररोहकारणं मां विद्धि सर्वभूतानां हे पार्थ सनातनं चिरन्तनम् । किञ्च, बुद्धिः विवेकशक्तिः अन्तःकरणस्य बुद्धिमतां विवेकशक्तिमताम् अस्मि, तेजः प्रागल्भ्यं तद्वतां तेजस्विनाम् अहम् ॥ १० ॥
In plain words
Know me as the seed of every being, Pārtha — the seed meaning the cause from which they sprout; and sanātana meaning ancient, there from the beginning. And more: I am buddhi in the intelligent — buddhi being the inner organ's power of discrimination, and the intelligent being simply those in whom that power is awake. I am tejas in the brilliant — tejas being boldness, and the brilliant being those who have it.
Close to the Sanskrit
Know me as the seed — the cause of sprouting — of all beings, O Pārtha, as sanātana, that is, ancient. Moreover: I am buddhi, the discriminating power of the inner organ, of the intelligent, that is, of those who possess the power of discrimination; I am tejas, boldness, of those who possess it, of the brilliant.