The Upanishads are a collection of texts which contain utterances (Sruti) concerning the nature of Ultimate Reality (Brahman) and describing the character of, and path to, human salvation (Moksha or Mukti). ‘Upanishad’ is made of Upa, Ni and Shad, literal meaning being ‘near, down, sit’. The actual meaning of the term is “to sit down with reverence near a Master” to learn the Truths about attaining the highest goal and purpose in life, Liberation from Illusion, Realizing one’s Real Nature. So they were passed on through Master-Desciple communications.
They are called Vedanta, the “last chapters of the Vedas”. The concepts of Ultimate Reality (Brahman) and the Soul or Self (Atman) are central ideas in all the Upanishads. “Know your Ātman” is their thematic focus.
More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads.