The concept called "tether" by Masters and Saints was a mysterious, but simple thing. It was a sort of imprint one could leave on the world by using their essence. The spot marked by the imprint was where one's soul was anchored to the world — Masters could only imprint themselves on the waking world, but Saints could imprint themselves on the Dream Realm, as well.
In addition to that, the tethers placed by Saints were much more expansive and deeply rooted in the fabric of the realm than those placed by Masters. In fact, Sunny was quite certain that they were entirely different things — it was just that both served the same purpose, so humans just used the same word for both.
The reason why Transcendent tethers were so much more robust than the Ascended ones was because of the nature of Saints. The soul of a Saint was connected to the world, and so, it interacted with the world much closer.