The first major part of the Vedas is called karma kāṇḍa. It describes how to live, how to follow the dharma in order to grow as a human being. The instructions of karma kāṇḍa produce a high-level art of living. Why? Because it is preparation for the knowledge of the upaniṣad, which makes the seeker realize his freedom.

The word upaniṣad can be interpreted in two ways. A deep, implied meaning (lakṣyārtha) and a more literal meaning (mukhyārtha or vācyārtha). This is typical of vedānta.

The deep meaning is as follows: Upa means that which is most 'near'. What is the closest? The seeker's self, ātma, which is identical with Brahman. The piece ṣad comes from the root 'sad', which means 'to destroy'. What is being discontinued? The revealed knowledge of the upaniṣad, if properly explained by a teacher, abolishes the ignorance of my true nature as consciousness. The suffix 'ni' stands for niścaya, it says that well-realized knowledge makes the person realize forever that he is pure consciousness. After knowledge, the jñānī is free forever. Then the definition becomes: That which is forever (ni, ignorance) removed (sad) from (my true nature) as that which is closest (the self, ātma). Not to liberate the words of the upaniṣad, but the self-realization of the deeper meaning of the words.

The literal meaning (mukhyārtha or vācyārtha) of the word upaniṣad is as follows: Close (upa) down (ni) to sit (sad) (with the guru, who explains the truth of upaniṣad).

Vedānta usually uses the ten upaniṣads, on which Ādi Śaṅkara commented (bhāṣyas), an explanation that is still used in modern vedānta today. These are called the mukya upaniṣad's. These are the ones with the Veda they are in:

ṛgveda: aitareya
sāmaveda: chāndogya and kena
yajurveda: īśāvāsya, taittirīya, bṛhadāraṇyaka, and kaṭha
atharvaveda: māṇḍūkya, muṇḍaka, and praśna

Every upaniṣad uses other methods (prakriyā) to attain self-knowledge. This completes the teaching of vedānta. Three reasons for this. Ignorance has many facets, so all kinds of things have to be dismantled. Secondly, it can differ from person to person which methodology suits her or him well. The third reason is that someone like Ādi Śaṅkara wanted to show that all methods of vedānta lead to the same consistent truth.

To understand and internalize the meaning of the upaniṣad you need a teacher who has been taught the meaning of the upaniṣad by her or his teacher.