In plain words
The fourth Veda is the Atharvaveda. Many people have described and explained it in many different ways. Some have not accepted its status as a Veda at all, while others have said that it is merely a remnant left over from the other three Vedas — that is what they have called this Atharvaveda. In the other Vedas, only the powers and capacities of the deities are set out. But this Veda's own special characteristic is precisely its teaching that, through the practice of yoga, a human being can accomplish certain wondrous feats by his own power and capacity — it is through this Veda alone that Aṣṭāṅga-yoga, Haṭha-yoga, tiraskaraṇī-vidyā, and the like, are accomplished. By a deity's grace, even feats that seem impossible can be achieved. We must therefore hold that the Vedas are of the very greatest importance, and that everything is contained within them. The Vedas alone are the roots of Bharatiya culture; these Vedas are apauruṣeya and self-existent.
Everyone must accept — there is no escaping it — that these are pre-eminent within world culture. Having been studied from teacher to pupil, mouth to ear, and preserved in unbroken succession, they remain alive and standing firm to this very day. No one at all has the power to alter them; this is an inviolable body of knowledge. Be that as it may, scholars have declared that the essence of all four Vedas is one and the same. These four comprise the karma-kāṇḍa and the jñāna-kāṇḍa. A human being can, and must, obtain his own puruṣārthas from these very Vedas. Because studying all four Vedas in full is held to be too difficult, the elders have, down the ages, conveyed the essence of the Vedas to us through the Smṛti, Purāṇa and Itihāsa texts, all of which derive their authority from the Śruti.
The karma-kāṇḍa and the jñāna-kāṇḍa stand in a mutual relation of effect and cause. This is because it is a natural and well-established fact that result follows in accordance with action, and that action itself arises in accordance with Jñāna. It is for this very reason that human beings obtain their results in accordance with their actions. For the practice of good action, Jñāna is indispensable. That Jñāna