In plain words
It is the Veda, too, that had people recite the mantras and rites fit for the progress of mankind in the world, for its lasting good and its welfare, gave them to it, rejoiced in them and made them flourish.
It is by doing karma that purity of citta is gained
The body is duality. The Ātma is non-duality. Only when the sense of being the body wanes does the radiance of the Ātma shine forth. Gathering in is natural to the body. Bestowing grace is the nature of the Ātma. As soon as the upādhi subsides, samādhi comes to us. Only when samādhi is reached does the supreme principle become clear to us. This samādhi is the sādhaka's highest goal. On the day one gains the Viveka that the body one believed for a time to be one's own is not one's own, on that very day one enters the supreme principle. The culmination of karma is Jñāna. The ground of Jñāna is karma. The union of karma and Jñāna is service. Hence “Cittasya śuddhaye karmaḥ.” It is by doing karma that purity of citta comes to us. It is through purity of citta that the Ānanda of the Ātma is gained. Every human being has no choice but to do, as his duty, the karmas he must do. Good deeds have to be done with a good mind. But the pride that says, ‘This fruit came about through my own karma,’ is not good. Nor is the attachment that says, ‘I alone must enjoy the fruit of the karma I did,’ a fitting one. Only when Ahaṅkāra in the doing and pride in the attachment are let go do renunciation and enjoyment take shape as one.
Karma-sannyāsa is not giving up karmas; it is giving up the fruit of karma
That is why the Teacher of the Gītā too declared that the one who sees inaction in action and action in inaction is the ‘yogi’. In the fifth chapter, on the subject of karma-sannyāsa, he taught that giving up karma is not what matters; what matters is giving up the fruit of the karmas one has done. We take only the acts done physically to be karmas. But that is not what the Teacher of the Gītā meant. Each one's body is itself karma. Sannyāsī or householder, when a person takes on a body, he comes bound up with karmas.