In plain words
is it not the Divine itself that keeps declaring this? In saying that varṇas depend on guṇa and karma, it becomes clear that a relation exists between guṇas and varṇas. So, if one thinks a little at length about the word 'varṇa' and inquires into it, it becomes easy to understand. Now, 'varṇa' means 'colour'; this meaning is very commonly used in the world. Everyone knows this much, but not everyone knows the meaning of its origin. Only when this is known does the true meaning of the word 'varṇa' become clear.
'Vṛ' means 'to explain' or 'to choose'. 'Rn' and 'ram' carry the meaning ramaṇa, 'delight'; there is also the sense 'to take delight'. So, 'to select and choose' means 'to accept with affection' -- that is how the meaning of the word 'varṇa' becomes known.
Now, among colours, white, red and black are the most important. The rest of the colours are merely mixtures of these three, not separate colours in their own right. White belongs to sattva-guṇa; red to rajo-guṇa; and black to tamo-guṇa -- this is the secret of creation, and it is natural to the world. As for the meaning of these words: living beings are born bound to whichever guṇas they favour, choose or desire, in accordance with those very guṇas. So varṇas are what beings have brought upon themselves out of their own desire; they were not devised by anyone else. This desire follows the jīva's own Buddhi; commonly, it is said that the intellect arises following desire. Following the intellect, karmas proceed; following those karmas, births arise.
'This indeed is what is meant by guṇa-karma-vibhāgaśaḥ.' While the Śruti and the Smṛtis so elaborately establish the causes of birth and death in matters of lineage, sect, caste, gotra and the like, human beings, unable to grasp this, make up explanations to suit their own wit and take delight in them -- what is that but sheer ignorance?