The Goddess is the tutelary religious figure of the Brotherhood of Mages who run the pocket universe that is Arth. Her worship seems to involve magical groves of trees with running water and peaceful protection in many locations around the world, or at least the Pentarchy.
The first we hear of Her is when the Mages of Arth begin mentioning the fact that she has been willing to protect a strange capsule from another world that suddenly appears in orbit around Arth. We then learn that somehow, the Brothers are required to maintain celibacy in Her name, even though everyone who can sense the vibes of Arth's blue stone walls and corridors recognize that the place is basically joyful, and really wishing that things would get more rhythmic and musical, please.
Later in the novel, the holy groves of the Goddess become key locations for a number of events, and the Goddess herself intervenes to require one of her worshippers to intervene in events of great evil that are going on against everything She stands for.