In plain words
is explained at great length in the Mahābhārata, in the Śānti-parvan; namely, whatever Brāhmaṇa, being fond of [sensual] enjoyment, falls prey to egotism, anger, desire and the like, such a person, being regarded as sāttvic mixed with rajas, came to be determined as a Kṣatriya — this is how it came about. Not all Brāhmaṇas are predominantly sāttvic and fond of karma, are they! Brāhmaṇas who become fond of untruth and, by nature, [predominantly] tamas mixed with rajas — such persons who deal in these two guṇas came to be called Vaiśyas. As for those remaining, who make their living through violent action and, being devoid of purity, live purely in conduct predominantly of the tamas-guṇa — such persons were determined to be Śūdras. In this way, in accordance with guṇa and karma, the Brāhmaṇas themselves arranged themselves into various varṇas and kept bringing about the protection of the world — this is the Śruti's own testimony.
Those who are purely sāttvic are called Brāhmaṇas; and those who conduct themselves with rajas as their predominant guṇa, endowed with valour and daring, are called Kṣatriyas; such persons may, through their strength and valour, bring about the protection of the world. Subsequently, those who, lacking either the nature of valour or the valour of arm-strength, but possessing only the strength of resourcefulness and skill in commerce, and having the inclination to properly employ those [skills], came to be designated as Vaiśyas — the class predominantly mixed with rajas and tamas. As for those remaining, who have neither the power of asceticism, nor valour, nor commercial aptitude, and conduct themselves [merely] — that is to say, those persons who are purely tāmasic and predominantly tamas-guṇa in nature — are designated as Śūdras and the like; they attain fulfillment by employing their own bodily labour for the sake of the welfare of the world.
The above four varṇas, not being separate and distinct from one another, having been demonstrated to be one and the same body alone, with no room for the distinction that one is superior and another inferior, and each remaining in its own respective place — every human being, conducting himself in a manner befitting his own station, may attain the supreme state — with this broad conception, this ancient Vedic system of varṇas