The Land of Light is a part of P3X-797. That planet is evidently in synchronous rotation with its sun, thereby making one side of the planet perpetually as bright as a summer day, and leaving the other side cloaked in eternal nighttime shade. The former is referred to by the locals as Land of Light, and the latter as the Land of the Dark.
The Land of Light had been plagued with disease, which the inhabitants believe to be a curse from evil gods, that slowly devolved those it infected into savages. When SG-1 and SG-3 came to their world, the locals presumed the explorers to be good gods. However, the plague came back to Earth and Stargate Command scientists were scrambling for a cure. Teal'c took an unwilling sample of blood from one of the people, which the doctors analyzed to have antihistamine, which repels the plague. Using darts filled with the drug, SGC troops were able to cure many of the Touched, including SG-1 member Dr. Daniel Jackson, and later taught their families to fight the plague. (SG1: "The Broca Divide")
The Land of Light is inhabited by the Untouched. The leader of the people is Tuplo, who offered the Tollans sanctuary in the Land of Light. However, this offer was refused by Omoc, who was deeply unimpressed by the Untouched's primitive level of development. (SG1: "Enigma")
Later, after Apophis' failed attempt to destroy Earth using Rya'c, Drey'auc and Rya'c fled to the Land of Light. They did not stay here very long; they moved to live elsewhere with other Jaffa. (SG1: "Family")