This is the name of the valley of the Arnon River. It creates the border between Moab and the Amorites. (See: [[rc://en/ta/man/translate/translate-names]])
The metonym "into your hand" means "into your control or power." Alternate translation: "I have given you the power to defeat" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Moses speaks to the Israelites as if they were one man, so the word "your" and the commands "begin to possess" and "fight" are singular. (See: [[rc://en/ta/man/translate/figs-you]])