To what should your eyes look?
Rashi: At what is Emes and straight.
R. Yonah: They should not be raised - "Rum Einayim u'Rchav Lev Nir Resha'im Chatas" (21:4), "v'Ram Levavecha v'Shachachta" (Devarim 8:14). Anyone who gets haughty, he stumbles in Aveirah (Avodah Zarah 18b). The heart and eyes are two merchants of sin (Yerushalmi Brachos 1:5).
Malbim: Einayim are open eyes. Afapayim are closed eyes; the Afapayim (eyelids) cover them. Oe must open his eyes in matters of the Nefesh, to look at Elokim and His deeds, so see His deeds and Gevurah and go in His ways. "Nochach" is directly opposite. Regarding Hashem it says Nochachah - "u'Nchochah Lo Suchal Lavo" (Yeshayah 59:14). You must look opposite Hashem's ways - "Einai Tamid El Hashem" (Tehilim 25:15).
Why does it say "Yaishiru Negdecha"?
Rashi: They should look at the straight, in order to straighten your path in front of you.
R. Yonah: Your eyes should recognize your level, and desire and request only what is proper for you.
Malbim: (a) In matters of the body and its desires, one must close the eyes and not veer after what they see. (b) "Negdecha" is opposite to the Yetzer and desires of the body and physicality.
