R. Yonah: To fear Me and love Me. The heart is desire; the heart should be entirely to Hashem. One should not yearn for anything other than His service. His worldly desires should be only for His Avodah.

Mesilas Yesharim (Perek 16): The heart is the king over all parts of the body, and conducts them. It should bring itself to Avodas Hashem. If it does not, the Avodah of the other limbs is nothing, for they go in the direction that the heart wants to go in.

Malbim: The laws of Chachmah will rule in his heart. They will be written on and engraved in it; nothing else will be found in it. Doubts of heresy and denial will not rule over it.

R. Yonah: They are Mitzvos and good deeds.

Malbim: The eyes represent desires and evil images that are based on the senses - "v'Lo Sasuru Acharei Levavchem" (Bamidbar 15:39) is heresy; "v'Acharei Eineichem" (ibid.) is Zenus. The eyes will guard Hashem's ways, and not be impressed with worldly lusts and the Yetzer's desire.