What does Joshua 13:9 mean?

Their territory started at Aroer, on the edge of the Arnon Gorge, and included the city in the middle of the valley, plus the whole plateau of Medeba as far as Dibon. Joshua 13:9 - Modern Text Bible

(The boundaries of the land given to Reuben, Gad, and half of Manasseh are described, starting from Aroer and Medeba.)