It can refer to two things:
* The Big Island of Hawaiʻi: This is the largest and youngest of the main Hawaiian islands, and it's often referred to as "Ka Moku Nui".
* A large, significant island: The term can also be used in a broader sense to refer to any large, important island.
It's worth noting that "Ka Moku" by itself means "the island". So, "Ka Moku Nui" literally translates to "the island that is big" or "the big island".