What do we learn from "Kol Ge Yinasei"?
Eruvin 54a: If he raises himself (becomes haughty), Hashem lowers him - "umi'Bamos ha'Gai" (Bamidbar 21:20). If he repents [from his haughtiness], Hashem raises him.
Rashi: Since all canyons will be raised, and "Kol Har v'Giv'ah Yishpalu", the entire land will be flat.
Radak: This is a metaphor. Ascending and descending mountains wearies travelers. They (who return to Yerushalayim) will not weary.
Malbim: Normally, when one wants to even out a path, and there are mountains, hills and canyons, they carry earth of the mountain to the valley. In the future, each canyon will rise by itself.
What is "he'Akov"?
Radak: It is a crooked path.
What are "ha'Rechasim"?
Rashi: This is like "v'Yirchesu Es ha'Choshen" (Shemos 28:28). 1
Radak: They are mounds on the roads 2 . It is an expression of height, like "me'Ruchsei Ish" (Tehilim 31:21).
Rashi (Shemos 28:28): These are mountains so close to each other that it very hard to descend to the canyon between them.
Malbim: Normally, when one wants to straighten a path crooked due to hills, first they level the hills to be flat, and then automatically the path is level. Then, miraculously, first the path will be straight, and afterwards the hills will be flat.
What is "Bik'ah"?
Rashi: It is a flat plain.
