Le Nui is a humongous jungle islanda and the place where the Le-Matoran originated from. It is currently hidden.
Le Nui was created by the Great Spirit Mata Nui, who then created a few hundred Le-Matoran and dropped them all onto the island. Vent, the leader of the Le-Matoran decided to set up a civilization and soon, Le Nui was filled with cities, villages and outposts.
After the dreaded Matoran Civil War on Metru Nui, the Makuta known as Levosk was assigned to here. Once the Brotherhood of Makuta was under Teridax's control, Levosk and the Le-Matoran of Le Nui hid Le Nui so that it would never be found.
So far the only ones who know where Le Nui is other than the Le=Matoran who live there are Levosk, Artahka, and the Order of Mata Nui.