What are "Sifsei Navon"?
Rashi: When one rebukes a Navon, he answers 'I sinned', like David said to Nasan "Chatasi" (Shmuel II, 12:13).
R. Yonah: Navon is one who understands a matter amidst another matter. Therefore, Chachmah is on his lips the entire day in all matters that arise, even though he did not learn about them in the way of Chachmah from someone else.
Malbim: A Chacham gathered the laws of Chachmah from his Rebbeyim and conducts according to them, without investigating to understand their reasons. A Navon investigated them to understand them and derive a matter amidst another matter. A Chacham's Chachmah is stored in his heart. In every matter that arises, he must take counsel with his heart if the act agrees with the laws of Chachmah that he received. A Navon understands the reasons. He already built 'buildings' [on the Chachmah]. The laws of Chachmah are fluent on his lips. Safah refers to external speech without thought at all. The Navon need not think at all, for it is always ready in front of him.
What is the meaning of "v'Shevet l'Gev Chasar Lev"?
Rashi: A Chasar Lev does not hear (admit that he sinned) until he is stricken, like Pharaoh.
R. Yonah: Chasar Lev, not only does he not know to understand on his own - even when they teach him, he does not understand. He is chastised only via a staff. The only solution for Naval (a base person) is via subduing and disgracing him.
Malbim: The heart is the power in the Nefesh that rules over the body, to conduct according to the laws of Chachmah. One who conducts opposite to the laws of Chachmah, he is called Ikesh Lev and similar names. One who has no conduct at all, and always does like crosses his Ru'ach - sometimes so and sometimes so, and there is no power that rules, not to the ways of Chachmah and not to the ways of folly. He is called Chasar Lev. He needs a staff to tilt him to the path. Man has choice; his heart leads him on the path. A horse or donkey has no heart that rules. An external force is needed to direct it - "Shot la'Sus Mesef la'Chamor [v'Shevet l'Gev Kesilim]" (26:3). Chasar Lev is like an animal, and needs a staff to force him, lest he pervert his ways and damage.
