What does the Torah mean when it states that Moshe "took the heads of the tribes"?
Rashi: It means that he drew them with words - 'How fortunate are you that you have been chosen to judge the children of Avraham, Yitzchak and Ya'akov, people who are termed "Achim, Re'im, Cheilek, Nachalah" and many other expressions of endearment!'
Why does the Pasuk omit "Nevonim"?
Rashi and Targum Yonasan: Because Moshe didn't find such men. Of the seven qualities mentioned by Yisro, Moshe found only men with three
What role did the officers (the policemen) play?
Rashi: Their job was to bind those who were Chayav Malkos and to administer the lashes
QUESTIONS ON RASHI
Rashi writes that Yisro gave seven qualities needed for judges. The Torah mentions only four - "Anshei Chayil Yir'ei Elokim Anshei Emes Son'ei Vatza" (Shemos 18:21), and the three that Moshe found are not among them!
Moshav Zekenim, Mizrachi: Yisro also gave the three qualities mentioned here; the Torah wrote them only here.