What is "Navu"?
Radak: The root is Avah. The Aleph is silent, and the Nun is due to Nif'al conjugation. The same applies to "Navu Lechayayich" (Shir ha'Shirim 1:10). It refers to beauty. Beauty of the legs is a metaphor, like "Mah Yafu Pe'amayich" (ibid., 7:2). How beautiful are the feet of the herald who will ascend the mountain to herald Shalom, good and salvation, and tell Tziyon "your G-d took reign"!
Will the herald truly ascend the mountain?
Radak: No. This is a metaphor - the matter will be known in the world as if people on mountains announce salvation. It is normal to announce good; one who is zealous to do so, he is praiseworthy.
Malbim: This is a metaphor, like it says above "Al Har Gavoha Ali Mevaseres Tziyon" (40:9).
Above (40:9) it says "Mevaseres Tziyon" (feminine). Why does it say here "Raglei Mevaser" (masculine)?
Rashi (40:9): If they merit, [salvation] will be fast, like a male; if they do not merit, it will be weak like a female, and delay until the deadline.
Why does it mention Shalom, Tov, Yeshu'ah and Malach Elokayich?
Malbim: First there will be Shalom from the enemy. After will be Tov (Binyan ha'Mikdash and the sprouting of Keren David), and then Yeshu'ah (the gathering of exiles). Afterwards, Malchus Shamayim will be revealed in all the land and in all nations.
Why will he say "Malach Elokayich"?
Radak: Until that time, they will not see that He is king. Then, all will say that He is king - "v'Haysah la'Shem ha'Meluchah" (Ovadyah 1:21).
