What are the Upanishads and the Vedanta? What is their philosophy?

The Vedas are the four sacred texts of the religions of India (often referred to collectively as "Hinduism"). "Veda" is a Sanskrit word meaning "knowledge." The Vedas were regarded by believers as having been revealed to humans through meditation.
Like the sacred texts of most world religions, the Vedas began as oral tradition. They were handed down verbally from generation to generation, beginning around 1400 BCE, and eventually written down. They are known as the Rig Veda, the Yajur Veda, the Sama Veda, and the Atharva Veda.
Each of the four Vedas has four parts: a Samhita (scared hymns) an Aranyaka (rituals), a Brahmana (commentary on rituals), and an Upanishad (meditation and philosophy of how humans are related to the universe).
"Vedanta" also came to refer to one of the six traditional schools of Indian religion, because they focused on the Upanishads.
The central teaching of the Upanishads is that everything in true reality ("Brahman") is unified and connected, and the appearance of individuality and separation (for example, when regarding two different people) is only an illusion. This is summed up in the phrase "Atman (a person's core self, or soul) is Brahman (the cosmos)." The purpose of meditation is therefore to overcome the illusion of separateness and realize the unity of all reality. This is known as "achieving enlightenment."
http://hirr.hartsem.edu/ency/Hinduism.htm

https://library.ucalgary.ca/religiousstudieswebguide