According to the Upanishads, the Atman is the true self, which is immortal, unchanging, and beyond the limitations of the physical body and the mind.

The Upanishads teach that the Atman and Brahman are one and the same. This is known as the concept of Advaita, or non-duality.

The ultimate goal of spiritual practice in the Upanishads is to realize the true nature of the Atman and to attain moksha, or liberation from the cycle of rebirth.

The concept of reincarnation in the Upanishads is closely linked to the law of karma, which states that every action has a corresponding reaction. The actions of an individual in one life determine the circumstances of their next life.

The Upanishads teach that the mind is an obstacle to spiritual realization because it is constantly fluctuating and changing. It is only by transcending the mind that one can experience the true nature of the Atman.

Maya is a Sanskrit term that refers to the illusion of the world. The Upanishads teach that the world we perceive is not real but is a product of our own minds. It is only by transcending maya that one can experience the true nature of reality.

The central message of the Upanishads is that the Atman is immortal and unchanging, the world is an illusion, and the goal of life is to attain moksha, or liberation from the cycle of rebirth.

The Brihadaranyaka Upanishad is considered to be the most important and influential Upanishad. It is one of the longest and most comprehensive Upanishads and contains a wide range of philosophical teachings.

'Neti neti' is a Sanskrit phrase that means 'not this, not this'. It is a method of negation used in the Upanishads to describe the true nature of the Atman. It means that the Atman cannot be described in terms of any positive qualities, as it is beyond all limitations and definitions.

'Tat Tvam Asi' is a Sanskrit phrase that means 'Thou art That'. It is a statement of identity between the Atman and Brahman, the ultimate reality. It means that the true self of every individual is identical to the universal consciousness.

The guru plays a crucial role in the Upanishads as a spiritual teacher who guides the disciple on the path to moksha. The guru is a source of knowledge and wisdom and serves as a role model for the disciple.

Karma is a Sanskrit term that refers to the law of cause and effect. It is the sum of one's actions and their consequences and determines the circumstances of one's next life.

Samsara is a Sanskrit term that refers to the cycle of birth, death, and rebirth. It is the source of all suffering and the goal of life is to break free from samsara.

Moksha is a Sanskrit term that refers to liberation from the cycle of birth and death. It is the ultimate goal of spiritual practice and a state of perfect peace and happiness.