Rashi on the Prophets and Writings

For lo, the sun comes This instance of יוֹם is an expression of sun, for so did the Sages state that there will be no Gehinnom in the future, but the Holy One, blessed be He, will take the sun out of its case; the wicked will be punished thereby and the righteous will be healed thereby. That is the meaning of what is stated (verse 20): “And the sun of mercy shall rise for you who fear My Name, etc.”
neither root nor branch Neither son nor grandson

and be fat an expression of fat, as in (Jer. 50: 11), “as you become fat, like a threshing heifer.”
as fatted calves [the calves] that enter the team to be fattened; kopla, cople in Old French: animals tied together.

And you shall crush and you shall press. This is an expression of pressing, similar to (Ezek. 23: 8) “they pressed their virgin breasts.”

that he may turn the heart of the fathers back to the Holy One, blessed be He.
through the children lit., on. He will say to the children affectionately and appeasingly, “Go and speak to your fathers to adopt the ways of the Omnipresent.” So we explain, “and the heart of the children through their fathers.” This I heard in the name of Rabbi Menahem, but our Sages expounded upon it in tractate Eduyoth (8:7), that he will come to make peace in the world.