In plain words
According to their own respective natures found in the world, human beings were divided into four kinds. These very kinds are Brāhmaṇa, Kṣatriya, Vaiśya and Śūdras. These are not merely a selfish, cunning device devised so that the privileged might suppress the unprivileged, nor an envy that obstructs human progress. It is best to regard it as a plan devised so that the whole multitude of humankind may attain development in accordance with their own nature and dispositions. This is the royal road to human progress. It is only steadfastness in one's own dharma and [prescribed] action, which forms the very shape of the harmony of life — that alone. In accordance with the division by guṇa and karma, the four — Brāhmaṇa, Kṣatriya, Vaiśya and Śūdras and the like — were created by Me, so the Teacher of the Gītā has already said.
'Cāturvarṇyaṃ mayā sṛṣṭaṃ guṇakarma vibhāgaśaḥ, tasya kartāraṃ api māṃ viddhi akartāram avyayam' — thus it is well known to readers that God Himself has declared that He, though the doer of these, is the non-doer and the unchanging one. The system of varṇas comes about precisely by [one's own] guṇa and karma. In the very beginning, this world, being purely predominant in the guṇa of sattva, all people remained as Brāhmaṇas alone. Subsequently, by [their] actions and by [their] guṇas, the varṇas came to be established. That one, single class, which existed as the class of ṛṣis, is the Brāhmaṇa class. Because differences of guṇa arose, it became necessary to divide [society] in this manner for the sake of social welfare — that the great ṛṣi Bhṛgu too answered the great ṛṣi Bhāradvāja so