Out of the portion of the children of Judah was the inheritance of the children of Simeon: for the part of the children of Judah was too much for them: therefore the children of Simeon had their inheritance within the inheritance of them.
The children of Simeon got their land from the children of Judah because the children of Judah had too much land. So, the children of Simeon's land was part of the children of Judah's bigger land.