Rashi: Just like when reaping standing grain, he gather sheaves with one hand, and the other hand detaches them with the scythe. So Sancheriv will uproot and exile everything, and even the sheaves that drop at the time of harvesting, he will gather and carry them. I.e. even the fleers will not escape his hand; he will gather them.

Radak: Just like one who harvests, he gathers the grain and reaps it together, and he leaves over only a little - one here and one here, so Melech Ashur will do to Yisrael. From the 10 tribes, only a few remained, scattered in the land, and also from them they will take and gather what they find. "Katzir" is an adjective, with the same grammatical form as Sarid and Palit.

Malbim: The verse is a metaphor for the three exiles via Sancheriv mentioned above (1:7-8, 8:23). The first time was like harvesting a field full of standing crop from Hashem's Brachah. The land was full of serene residents.

Rashi: The matter is repeated in different words.

Malbim: The second exile via Sancheriv was like reaping scattered sheaves. The land was empty due to the first exile.

Rashi: It is the canyon facing Yerushalayim. In Yehoshua (15:8) we find Emek Refa'im close to Yerushalayim. Radak - perhaps giants (Refa'im) lived there.

Radak: The enemy will gather Yisraelim who flee, like one who gathers sheaves - one here and one here. He will do so in Emek Refa'im and other places. It mentions Emek Refa'im because it was close to Yerushalayim, and Melech Ashur seized everything from what he captured; he was outside ¹ Yerushalayim.

Malbim: The third exile via Sancheriv was like reaping in a thin (unfertile) place that grows few sheaves. One who comes to gather them cannot use a scythe; he just gathers one by one. So he found only scattered remnants, and exiled them.