Those were the shares of land Moses had given the eastern tribes when he was in the plains of Moab. The plains are across the Jordan River east of Jericho.
It includes half of the land of Gilead. It also includes Ashtaroth and Edrei. They were the royal cities of Og in Bashan. That land was given to half of the family line of Makir. He was the son of Manasseh. Each family group received its share.
Moses had given two tribes and the other half of the tribe of Manasseh their shares east of the Jordan River. But Moses had not given the Levites a share among the other tribes.