Tollan are members of a technologically advanced human civilization. They shared their technology with a less-advanced neighboring world, but that world was not prepared to use that technology wisely and ended up destroying themselves. When their world was destroyed, it affected the orbit of the Tollan homeworld and that world, too, was destroyed. The Tollan were able to escape the death of their civilization by finding a new homeworld, Tollana, via starships, but they eventually built their own stargate. Tollana was destroyed by the Goa'uld and we lost contact with them, not knowing if any survived. We first met the Tollan in the episode, 1.17 "Enigma".