In plain words
Arjuna! In you the four things are ready. What are these four things? The lamp bowl is Vairāgya, the oil is Bhakti, the wick is mastery of the senses, and inquiry into the Ātma is the fire in it. Kṛṣṇa, who lights them, is right at hand. Precisely because the four qualities are there, you are worthy of the teaching of Dharma and of gaining Jñāna.
The Creation of the Varṇas in the World
For this very end, that such a flame of Jñāna may shine in the world forever, I create the four varṇas. Why? So that social Dharma, the Dharma of the universe and the Dharma of Īśvara may be upheld in good order, I create the four varṇas. These four varṇas bear the names brāhmaṇa, kṣatriya, vaiśya and śūdra. Those who carry forward the eternal śāstra that is Satya are the brāhmaṇas. He who offers his body for his country is the king. That is why they are kṣatriyas. Those who have wealth and grain and shine with the virtue of Dharma are the Ārya-vaiśyas. Those who live in comfort with farming as their calling are the śūdras.
That is, the brāhmaṇas are the head, the kṣatriyas the shoulders, the vaiśyas the thighs, the śūdras the feet. For the protection and the guarding of Dharma, Kṛṣṇa laid down the principle that these four must each carry out their own duties. He gives them a power by which each part, while doing its own duty, helps too in the duty another part carries out. Kṛṣṇa showed that whatever danger befalls a clan, a community or a nation, the other limbs suffer too; they grieve. To carry out Dharma: that alone is their duty.
If a Thorn Pricks the Foot, the Eye Weeps
As we walk along a path, a thorn comes into sight. At once, through the bond within, the eye tells the foot, and the foot steps over it; seen another way, if a thorn pricks the foot, the eye weeps. In just this way Kṛṣṇa sets forth the bond between the four varṇas.
When the thighs or the shoulders are in pain, the face suffers. Whatever pain befalls the face, the legs suffer. Building up this mutual bond between the body's limbs, we must protect the Dharma of Īśvara. These varṇas were not set up for anything else, not for quarrelling, “I am great, you are small,” nor for making war on one another.
In the feet, in the thighs, in the shoulders, there is one single heart. One single blood is flowing. In limbs through which one blood flows, there is no room for an air full of hatred. So the brāhmaṇas are not a separate clan. Brāhmaṇas is the name given to the face. Those who protect the country are the kṣatriyas. What bears the head and the shoulders is the thighs. That is, the meaning is that the vaiśyas are those who give food to the kṣatriyas and the brāhmaṇas and stand by them as their support. None of these can stand without the support of the feet.
Not understanding it this way, they quarrel in all sorts of ways, Arjuna! You must recognise the inner meaning of the varṇas and take it on yourself to teach it to the world. That is, Jñāna, power, strength, creation: all of it is God's will alone. This boat of Jñāna alone carries man, tormented in the ocean of saṃsāra, to the farther shore. This very thing Kṛṣṇa called the fire of Jñāna. The fire burns Ajñāna to ash and bestows divinity.