What does this Mizmor discuss?
Rashi: It is based on Har Tziyon and Yerushalayim. Radak - it discusses their praise.
Radak (7, from Shocher Tov): Notzerim say that it refers to their savior; he was formed and born in Tziyon. There are many refutations of this.
Malbim: It discusses the praise of Tziyon and the nation in it.
Why does it say "li'Vnei Korach"?
Refer to 42:1:1:1-4 and the notes there.
What are "Harerei Kodesh"?
Rashi: They are Yerushalayim and Tziyon.
Why does it say "Harerei" (plural)?
Radak #1: Har Tziyon is not the same as Har ha'Moriyah, i.e. Har ha'Bayis.
Radak #2: Is due to the mountains near Yerushalayim - "Yerushalayim Harim Saviv Lah" (125:2). One of them is Har ha'Zeisim. It mentions Tziyon, for it is the place of the kingship and includes Yerushalayim - "Ir ha'Elokim Selah" (verse 3).