What does 1 Chronicles 5:8 mean?

The descendants of Reuben lived in Aroer, as far as Nebo and Baal Meon, and also in the land east of the Jordan River, spreading out as far as the edge of the desert that borders the Euphrates River, because their livestock had grown so numerous. 1 Chronicles 5:8 - Modern Text Bible

(This describes where the Reubenites settled, emphasizing how their need for pastureland led them to spread out.)