The Realm of Light (光の世界 Hikari no Sekai?) is a Plane of Existence made up mostly of light and makes up almost half of the Kingdom Hearts universe, similar to the Realm of Darkness. Like the Realm of Darkness, the Realm of Light has a Keyblade, notably the Kingdom Key, which chose Sora as its wielder. Most of the game's worlds are found here, save for those in the Realm of In Between, like Twilight Town, Castle Oblivion, and The World That Never Was. It can be presumed that most of the Worlds accessible by the Gummi Ship are either within the Realm of Light, or are related to it, like Twilight Town.
In Kingdom Hearts II, one notable citation has been made by Sora regarding the Realm of Light during the scene of Sora and Riku at the "end of the road" after defeating Xemnas: The other side... The realm of light is safe now. Kairi, the King and the others are there.