Vidya leads to liberation, while Avidya binds one to the cycle of birth and death (Samsara). The Mundaka Upanishad beautifully describes this in the verse: "Dve Vidye Veditavye Parā caivāparā ca" ...
The Upanishads that you can study in the website are Aitareya, Ishavasya, Karika, Katha, Kena, Mundaka, Mandukya, Prasna, Brihadaranyaka, Shvetashvatara and Taittiriya. Link - You can read and study ...