Albert Barnes Commentary 1 Kings 3:9

"Give thy servant therefore an understanding heart to judge thy people, that I may discern between good and evil; for who is able to judge this thy great people?" — 1 Kings 3:9 (ASV)

One of the primary roles of an Eastern monarch has always been to hear and decide legal cases. For this reason, supreme magistrates were naturally called “judges.” (See the introduction to the Book of Judges.) In the minds of the Jewish people, the roles of “judge” and “prince” were always closely connected, as their chief civil governors constantly took direct responsibility for judging legal cases (Exodus 18:16, 22; 1 Samuel 8:20; 2 Samuel 15:2–6).

Good and bad—that is, “right and wrong,” “justice and injustice.”