This sentence comes from the Song Dynasty poet Wang Zhongwei's Canggou Changfeng.
Canggou Changfeng: Shallow joy is like Canggou, and deep love is like Changfeng. Love is separated by mountains and seas. May the mountains and seas be even.
the meaning of this poem is: a shallow love for a person, like a white cloud in the sky, may disappear at any time; Have deep love for a person, just like the gentle wind, and accompany him in obscurity at any time and place. The person I love and I are separated by overlapping mountains and deep seas. Although the mountains can't be eliminated and the sea can't be filled up, I will still silently guard her like Changfeng.
Interpretation: Clouds are tangible, but they will dissipate at any time. It's as if you like to say them casually, but these sweet words may be insincere. Changfeng is invisible, but it will never disappear. It's like loving someone deeply, without romantic love words, but with sincere companionship and affection. Love cannot be seen with eyes. If you want to know whether he is your "pale dog" or "Changfeng", please remember to feel it with your heart.
Extended information
The meaning of the two images of white clouds and pale dogs:
"White clouds"? Is the "pale dog"? "Canggou" is "Baiyun". "Baiyun" is the name and expression of modern people to the clouds on the sky; "Canggou" is the ancient name for clouds. Nowadays, the idiom "white clouds and pale dogs" is used to describe that everything in the world is as changeable as white clouds in the sky.
In his poem Alas, Du Fu, a Chinese poet in the Tang Dynasty, said, "Clouds in the sky are like white clothes, but they must be changed like pale dogs." Therefore, modern people also use "white clouds and pale dogs" as "white pale dogs". Both idioms have the same meaning, and both use the vagaries of white clouds to refer to the vagaries of the world.