In plain words
The second, the Āraṇyaka, is made of both prose and verse. In it the part on karma is larger. Because a man gave up the life of the householder and entered vānaprastha, and the vānaprastha entered the forest and studied it there, it came to be called the Āraṇyaka. Within this itself, they named the last part the Brāhmaṇa. It dwelt at length on the path of Dharma alone, and went on setting out the form of yajñas, yāgas and the like.
The third is the Upaniṣad. In it they went on laying down certain paths for every man to follow in daily life, for the sake of Dharma, artha, kāma and Mokṣa. But it is only through vidyā and Viveka that one can attain this, an Upaniṣad. Among these vidyās there are two kinds of vidyā. One is para-vidyā. The second is apara-vidyā. All the Vedas went on teaching only the apara-vidyās. They explained only Dharma, artha and kāma. After that, they tried to make it known that the right path to Mokṣa is one's own effort.
The Vedas: na karmaṇā na prajayā tyāgenaike amṛtatvamānasuḥ. They went on showing that it is by this tyāga alone that Mokṣa, which is called immortality, is attained, and that without tyāga no one can reach such a state. So the Vedas went on teaching well only three of the puruṣārthas. The fourth, Mokṣa, comes to man by the grace of God and by man's own sādhanas. For this world the principle of both, the worldly and the spiritual, is most necessary. 'To be happy here, hematāraka-vidyā; to be happy hereafter, Brahma-vidyā,' they said. The means of living is one thing. The supreme goal of life is another. The Vedas went on teaching the principles laid down for the means of living. The Vedas go on teaching the fitting means to fulfil what one wants and to keep away what one does not want. From ancient times this has gone on growing through the line of guru and disciple.
The Veda has nine names: Śruti, anuśruti, trayī, āmnāya, samāmnāya, chandas, svādhyāya, āgama, nigama. We must try to understand well what each of these means. Śruti. The disciple utters with śraddhā and Bhakti the words the guru utters; in just the way the guru uttered whatever sounds he uttered, he hears them and utters them again; to this state they gave the name Śruti. That is, because he hears the guru utter it, utters it himself just as the guru did, and so studies that Veda, it is called Śruti and anuśruti. The Veda was infinite and remained one alone. Later they went on dividing it into three only: that is, the Ṛg Veda, the Yajur Veda and the Sāma Veda. Only these three went on spreading in the world. So it rightly came to be called trayī, as the form of the three Vedas. The root mna has one meaning, "practice".