Rashi: They are words of nobility and importance.

Malbim: He explains how Hashem revealed Chachmah and gave it to people via the Torah that He gave to His nation. There are four aspects. In every Mitzvah there is Dibur and Amirah. (a) Amirah is the expression in which the Mitzvah was said in the written Torah. (b) Dibur is the elaborate explanation of the Mitzvah that was handed over orally. Wherever it says "va'Ydaber Hashem El Moshe Leimor", va'Ydaber teaches that He spoke at length and verbally informed him details and Halachos of the Mitzvah. Leimor is the words of the Mitzvah in written Torah. The other aspects are (c) reasons for Mitzvos and their secrets, and (d) hidden secrets that were received, e.g. secrets of Torah, Ma'ase Bereishis and Ma'ase Merkavah. Each of these has two levels. "Aleichem Ishim Ekra v'Koli Es Bnei Adam" (4) - elevated matters for great people and Chachamim, and simple matters for commoners. Negidim refers to oral Torah, which is the majority. A Nagid is one appointed over the nation. He commands to do. Just like a Nagid is the greatest and most esteemed in the nation, so all of them are great matters and fixed Halachos handed over to the leaders of the nation and their teachers. This corresponds to "Aleichem Ishim Ekra."

Malbim: Safah refers to simple, revealed speech. Miftach ha'Safah is the beginning of simple speech which is for commoners who do not understand great matters. It is Meisharim - straight matters that Sechel accepts, Everyone sees their straightness and understands them with straightness of heart.