Trion is the home planet of the Masters. It is known to have a stronger gravitational pull than Earth. It is also considerably hotter and more humid, most probably swamp-like. Like Earth, Trion has liquid water on its surface and supports water-based life, including the Masters themselves. No information on any other species living on the planet has been given, except for a moss-like plant that the Masters even took with them on their journey to Earth.
The TV Series added information about Trion that was not present in the books, including its name which was never revealed in any of the novels. According to West 468, Trion is part of a triple star system that never leaves any part of the planet without light. There is thus no night on Trion which is the reason why Masters need no sleep.