1)
Why does it say "va'Yamter" (an expression of rain)?
1.
Radak: A multitude came, like rain. This was wondrous!
2.
Ba'al ha'Turim: This is like it says "va'Yamter Hashem Barad" (Shemos 9:23). Just like hail struck Egypt, so the Slav fell on Resha'im and struck them.
3.
Malbim: Another miracle was that when they came to Machaneh Yisrael, they did not fly away; they came straight down, like rain.
2)
What is "She'er"?
1.
Rashi: It is meat. Radak - the matter is repeated in different words.
3)
What is "Ohf Kanaf"?
1.
Radak: It did not have strength to fly to and fro; Hashem made them fall on the Machaneh.