What does Numbers 31:8 mean?

They killed the five kings of Midian: Evi, Rekem, Zur, Hur, and Reba. They also killed Balaam, son of Beor, with the sword. Numbers 31:8 - Modern Text Bible

(The leaders of Midian are specifically named, showing the thoroughness of the victory. Balaam, who had previously tried to curse Israel, is also killed, tying up a major thread in the story.)