The Material Plane
The Progenitor Dragons are Siberys, Khyber, and Eberron. According to the creation story related by the ancient Giants, the three dragons created (or discovered) the Draconic Prophecy. Siberys and Khyber began to fight over it, and in the ensuing battle Kyber killed Siberys. Eberron then used her own body to bind and trap Khyber in the depths of the world, and at the same time restored life to the world.
All three, Siberys, Eberron, and Kyber, constitue the Material Plane.
Siberys, The Dragon Above, is the name given to the stars.
Eberron, The Dragon Between, refers to the main, physical world.
Kyber, The Dragon Below, is the massive subterranean realm found below Eberron’s continents and seas.