The first sentence is that the road of Taihang Mountain is difficult to walk, and no matter how strong the car is, it can't stand the toss after driving on the road for a long time. This is a metaphor of Fu, so the second sentence is not difficult to understand. Such a rugged and steep road, compared with people's hearts, is flat, which shows the viciousness of people's hearts, and also reflects the cold world and the rusticity of people's hearts.
These two poems are juxtaposed, so the second sentence can also be understood along with the explanation of the first sentence.