What does this Mizmor discuss?
Rashi: It refers to Churban ha'Bayis. Tehilim 14 above is like this - "Amar Naval b'Libo Ein Elokim", just it refers to Churban Bayis Rishon, and this is about Churban Bayis Sheni.
Radak (2): It is like Tehilim 14, which discusses Galus, just the words are slightly different. It begins with Galus and concludes with redemption. He put it between the Mizmor about Do'eg and that about Zifim, for a parable of David to his son, Mashi'ach. Just like David hoped for kingship, and also the good in Yisrael hoped for his kingship, and there were bad people plotting to kill him, e.g. Do'eg and the Zifim
Radak: So are Yisrael today! In every generation, the nations plot to eradicate us and mock our hope for Mashi'ach ben David - "Asher Cherfu Oyvecha Hashem Asher Cherfu Ikvos Meshichecha" (89:52), and when he comes they will try to nullify his kingship.
What is "Machalas"?
Rashi #1: It is the name of a musical instrument.
Rashi #2: It is for the illness of Yisrael when the Bayis will be destroyed.
Why does it say "Maskil l'David"?
Malbim: After the miracle of Sancheriv, they said that David hinted to it in Tehilim 14; he foresaw it with good Sechel, i.e. Ru'ach ha'Kodesh.
