Rashi

Every valley shall be raised and the mountain shall be lowered, thus resulting in a smooth, even, and easily traversed road. and the close mountains Heb. רְכָסִים, mountains close to each other, and because of their proximity, the descent between them is steep and it is not slanted, that it should be easy to descend and ascend. ([The word] רְכָסִים is translated by Jonathan as ‘banks,’ an expression of height like the banks of a river.) close mountains Heb. רְכָסִים. Comp. (Ex. 28:28) “And they shall fasten (וְיִרְכְּסוּ) the breastplate.” a champaigne Canpayne in O.F., a smooth and even terrain.