It has been sung of in many ways by the seers, in various distinct hymns, and also in the reasoned, conclusive words of the Brahma-sutras.
Explanation
Krishna has just promised to explain the field (kṣetra, the body and its makeup) and the one who knows the field. Here he says this teaching is not new: seers have expressed it in many ways, in the varied Vedic hymns (chandas) and in the reasoned, decisive statements of the Brahma Sutra (brahmasūtra-pada). Some commentators take 'brahma-sutra-padas' as the Vedanta text of that name, while others read it more generally as Upanishadic statements about Brahman.