What is the comparison to Chelev and Deshen?
Radak: Just like the Nefesh desires fatty foods, which everyone enjoys, so it will be pleasant to me. Chelev and Deshen are different words for the same matter.
Malbim: They satiate the body, and they are food for it. So my Nefesh will be satiated. I will give to it spiritual food three times a day, just like people eat physical food three times a day 1 .
In the days of Chazal, people ate twice a day; to honor Shabbos, they ate three meals on Shabbos! It seems that already in Malbim's time, it was common to eat three times a day. (PF)
What will satiate his Nefesh?
Radak: The songs that he will sing. "V'Sifsei" is like Ka'asher Sifsei. The same applies to "Im Lo Yisba'u va'Yalinu" (59:16; it is like Ka'asher Yalinu).
Malbim citing the Kuzari: It is Tefilah. Just like the body cannot survive without food, so the Nefesh needs Tefilah, to cling to its spiritual source.
What are "Sifsei Renanos"?
Rashi: They are languages (expressions) of song. This is like "va'Yhi Kol ha'Aretz Safah Achas" (Bereishis 11:1); the Targum is Lishna Chad.
Radak: They are words of song, like "Safah Achas u'Devarim Achadim" (Bereishis 11:1).
