The Upanishads are a collection of ancient Sanskrit texts that are part of the Hindu Vedas and focus on philosophical and spiritual concepts. They discuss six key concepts: Brahman is the ultimate reality and single transcendent principle of the universe; Atman is the eternal soul or true self within each person, which is identical to Brahman; karma and samsara refer to the cycle of death and rebirth governed by the law of karma; avidya is ignorance of the true nature of reality and self; and moksha is the ultimate state of liberation, enlightenment, and freedom from samsara achieved by realizing the identity of Atman and Brahman.