The meaning of this sentence should be combined with the full text, that is:
He forced him to be strong, the breeze blew the hills, he passed by, and the bright moon shone on the river.
It means that no matter how strong and fierce the enemy is, if you can stand still like a hill (that is, have peace of mind), even if the breeze is strong, it will not blow; If you can be calm as a river and calm as water, even if the moonlight shines, it will not affect your mood.
In the Book of Unification, Zhang Wuji couldn't understand this sentence at first after he got the True Classic of Jiuyang. Later he understood that the meaning of this sentence lies in the contest between the two sides. The key is not how profound and ingenious the opponent's martial arts are, but whether his heart is disturbed by his opponent's martial arts.
To put it bluntly, the competition of martial arts lies in one's own mentality, not the disparity of martial arts between the two sides. If you have a firm mind, no matter how good your opponent's martial arts are, you can get away from the enemy.
This sentence later appeared in Feng Ge's martial arts novel Kunlun, which was made by a young monk named Peanut after an epiphany.
Generally speaking, although this sentence was invented by martial arts novels, it belongs to an idealistic point of view. I tend to have the meaning of Zen and Buddhism.