Bright Moon is the base for all princesses. Bright Moon is a beautifully tranquil place, with its gigantic runestone decorating the front. The only thing that protects Bright Moon from The Horde is the Whispering Woods, thus the goal of the Horde is to take over that place. It is the the main setting of She-Ra and the Princesses of Power.
The Moonstone is one of the six known Runestones in Etheria. It lends power to princess Glimmer and Queen Angella. It is located in Bright Moon, it provides light for Bright Moon, it is near the edge of the Whispering Woods. The Horde are trying to reach Bright Moon and this stone is one of the things protecting Bright Moon from the Horde.