From the introduction:
The world of Kamigawa, positioned far from any other world known to the Walkers, is governed by the interplay between the mortals and the kami, minor gods or spirits of the world. However, when a child of the supreme kami, O -Kagachi, was stolen away, the physical and spiritual worlds gird for war against each other. The Kakuriyo, sometimes called the Reikai, is the spirit world where the kami dwell; its other half is that of the Utsushiyo where mortals live. Together they form a great sphere that makes the whole of the world.