Flee, save your lives; literally, your souls. The prophet's human feeling prompts him to this counsel; but he knows full well that a life of abject misery is the utmost that can be hoped for. And be like the heath in the wilderness; literally, and (your souls) shall be like destitute ones in the wilderness.
Imagine the ease of one who has been robbed of everything, and left alone in the desert; not less miserable is that of the Moabite fugitives. The word rendered "the heath" (‛arō‛ēr) is either miswritten for ‛ar‛ar, which occurs in the sense of "destitute" in Jeremiah 17:6 (see note), or also a rare plural form of the same word.
The sense remains the same. It is tempting to see an allusion to one of the towns called Aroer (as in Isaiah 17:2). But the only Aroer the prophet could be thinking of is that on the Amen (Deuteronomy 2:36), which could not be described as "in the wilderness."