What does Joshua 12:2 mean?

First was Sihon, king of the Amorites, who ruled from Heshbon. His kingdom stretched from Aroer on the edge of the Arnon Gorge, through the middle of the valley, half of Gilead, up to the Jabbok River, the border of the Ammonites. Joshua 12:2 - Modern Text Bible

(Sihon controlled a large area in the central Transjordan, and his defeat opened up the region for Israel.)