What does Deuteronomy 3:16 mean?

To the tribes of Reuben and Gad, I gave the land from Gilead down to the Arnon Gorge (with the middle of the gorge as the border) and as far east as the Jabbok River, which borders the Ammonites. Deuteronomy 3:16 - Modern Text Bible

(This verse details the specific boundaries of the land given to Reuben and Gad, marking out their territory.)