The Mahābhārata, considered as the ‘Pañcama-veda’ or the fifth Veda, has enriched human life as also itself by bequeathing the Bhagavadgītā to the posterity. However, the same epic also contains several other pieces of religious discourses, each one of them with a different name, but with word ‘gītā’ affixed to it. Of the sixteen such gītās, the Brahmagītā also is one. It comprises just nine verses, from the 4th to 12th verse, in chapter 35 of the Anuśāsanaparva.
In the course of his long discourse to Yudhiṣṭhira, the eldest of the Pāṇḍava princes, Bhīṣma, the grandsire, narrates how in the beginning of this cycle of creation Brahmā, the Creator, brought forth the four varṇas or classes of human beings—the brāhmaṇas, the kṣattriyas, the vaiśyas and the śūdras—and taught them wisdom.
The brāhmaṇas were advised to live a life of self-control, to study the Vedas with devotion, to offer sacrifices to the gods in heaven and not to hurt any being. They could get whatever they wanted, just by tapas (austerity) and jñāna (know-ledge). He also exhorted them to never stoop to performing the works meant for the śūdras. That would degrade them spiritually.
Since dharma was preserved by such brāhmaṇas, he counselled the other three classes to protect and serve the brāhmaṇas.
Because this teaching was delivered by Brahmā, it has been named as the Brahmagītā.
There is a Brahmagītā in the Sūtasaṁhitā which itself is a part of the Skāndapurāṇa. See SŪTASAṀHITĀ for details.