What does Deuteronomy 3:12 mean?

We took over this land at that time. I gave the tribes of Reuben and Gad the territory north of Aroer by the Arnon Gorge, including half the hill country of Gilead and its towns. Deuteronomy 3:12 - Modern Text Bible

(Moses explains how the land was divided among the tribes, with Reuben and Gad getting a large portion east of the Jordan.)