Nod is a region located on the east of the Garden of Eden. This was the region that Cain was exiled to wander in after he murdered his brother Abel. This may be a specific land or could have been the general region (east of Eden) that Cain wandered in. It is only mentioned once in the Bible, in the Book of Genesis along with the account of Cain's life. In Genesis the word Nod literally means land of wandering.
"Nod" (Hebrew: נוד) is the root of the verb meaning "to move to and fro" or "to wander." It describes one who flees from danger, or even one who is nervous or upset. As in the case of many place names, this place many have been attached well after the activity (wandering) that happened there.
Nod was a region located east of the Garden of Eden, possibly a specific area or just a general area of Cain's wandering. The land may have been inhabited before Cain's exile there, but it received its name due to the curse upon Cain.
After Adam and Eve were banished from Eden, God placed cherubim on the east side. These cherubim were placed to prevent Adam and Eve from re-entering the Garden and having access to the Tree of Life. With these cherubim placed on the east side of Eden, mankind had spread out eastward from Eden.
Its likely that Adam and Eve's children after Cain settled the area that would eventually be known as Nod. As God commanded, the couple would have continued to multiply and sent their children to fill the Earth. To do this the offspring would have spread out farther and farther East of Eden to fill the Earth. This would explain Cain's concern for someone killing him in the Land of Nod.
Land of Wandering
After Cain murdered his brother Abel, God sentenced him to be a wanderer on the Earth. The former blessings upon his work in the fields disappeared, forcing him to wander from place to place. Since the curse was specific to Cain, it is unlikely the entire region would have suffered.
As a lone wanderer, he would have had to depend upon God to show him where to find food and shelter. Feeling himself unfairly punished, Cain complained that not only lose God's blessings, but that other people would want to kill him because of what he had done. To assuage his fears, God marked Cain in a special way that would be a warning to anyone seeking revenge.
Though most of mankind at the time still revered the Creator, Cain, his wife and perhaps a small portion of his own family set out into the wilderness. Contrary to the curse, they would settle down at a safe distance in Nod, the Land of Wandering.
City of Enoch
By the time he was banished, Cain was about 130 years old. In all likelihood, he had had many children and one or two generations of grandchildren by then. It is not clear from the text whether any of these would have followed him and his wife into Nod. Over the years, though it had become necessary to build a city (Heb: ע י ר from a root meaning "open") — a place needed security watches. In whatever manner, Cain's family grew, with the son born to him at this time being named "Enoch," meaning to dedicate, or to inaugurate. This name became suitable for the settlement, the first recorded city: Enoch. A new era had begun for mankind.