The Great Barrier was an incredibly huge underground dome of rock and solid Protodermis that housed Metru Nui and the entire Silver Sea. It was here that the Toa Metru defeated Teridax in the Silver Sea.
During the Great Cataclysm, The Toa Metru escaped Metru Nui with a few Matoran through a hole in the Great Barrier, re-entered through the barrier to get the rest, and exited again to take them to Mata Nui.
The "ceiling" of the dome had two "suns" were actually holes in Mata Nui's surface. These holes were actually the Great Spirit's eyes, though they allowed sunlight to pass through them. After the Great Cataclysm one hole was covered up, but later uncovered by the Bohrok. There were also several gates in the barrier that connected them to Metru Nui.