From the Super Mario Wiki
The Light Realm is a region in Yoshi's Safari. It is the first of two worlds in Jewelry Land. It contains more cheerful, positive, and brighter levels than its darker counterpart, the Dark Realm. The Light Realm was formed as a result of a large earthquake caused by the removal of the twelve gems. King Fret and seven of the gems are captive in the Light Realm. It contains seven levels, each guarded by a Koopaling. After clearing the Light Realm, Mario and Yoshi save King Fret and travel to the Dark Realm.