What is the exact meaning of "Vayacharem es'hem ve'es Areihem"?
Rashi and Rashbam (both on Pasuk 2): 'They killed the inhabitants and declared all the booty Hekdesh'. 1
Ramban #1 and Seforno: They vowed that, when they entered Eretz Cana'an, they would destroy their towns (or donate all their property to the treasury of Hekdesh - Ramban), which they subsequently did. 2
Ramban #2: They destroyed the king of Arad's army and called the location of their victory Chormah; 3 and when they crossed the Yarden, Yehoshua killed the subsequent king of Arad together with the other kings of Cana'an, and when Yehudah entered the towns of the king of Arad, they declared them Hekdesh and called them Chormah, in fulfillment of the promise that they made here.
Targum Onkelos and Targum Yonasan: They killed the inhabitants and destroyed their towns.
See Sifsei Chachamim. And this explains why the Torah does not tell us which tribe inherited Arad (See Ramban)
Ramban and Seforno: See Shoftim, 1:17 and Ramban DH 'Vehishlim'. And the Torah is informing us here what they were destined to do in the future, as it does on a number of occasions (See Ramban Ibid).
See Ramban DH 'Gam Nachon'.