Concise Bible Dictionary:
This is mentioned only in Joel 3:2,12, as a place where the nations will be judged for their treatment of God’s people, when Judah is being brought into blessing. The name signifies “Jehovah is judge,” and some think that it is used symbolically for the place where God will judge. In Joel 3:14 occurs “the valley of decision,” which is also connected with God’s judgments.
Others however associate the above with the valley lying between Jerusalem and the Mount of Olives, now called the Valley of Jehoshaphat; but no trace of this name as attached to that particular valley can be found earlier than the fourth century. It was and is otherwise known as the Valley of the KIDRON.