What does Jeremiah 31:40 mean?

Every part of the valley, all the way down to the fields where ashes are dumped and the Kidron stream, right out to the corner of the Horse Gate on the east, will be set apart as holy to the Lord. Nothing in that whole area will ever be torn down or destroyed again. Jeremiah 31:40 - Modern Text Bible

(God promises that even the most neglected and unclean places around Jerusalem will be made holy and safe forever. The city will never again be ruined or defiled.)