The Upanishads are ancient Sanskrit texts that discuss the nature of Brahman, the ultimate reality, and Atman, the true self. They outline six main concepts: Brahman is the impersonal absolute that pervades all reality; Atman is the pure consciousness within all beings. Avidya is the ignorance that leads people to identify with their bodies and minds instead of their true selves. This causes samsara, the cycle of rebirth fueled by karma. Spiritual practices like meditation and moral discipline can lead to moksha, liberation from samsara by realizing one's identity with Brahman-Atman.