What does Bai Juyi mean by "flowers are not flowers, fog is not fog, come at midnight and leave at dawn"?

Imagine that before midnight, the poet suddenly woke up, put on his gown and walked slowly to the window. At this time, the moon is closing, and my thoughts can't help wandering. It's just that scene won't go away. Is it flowers or fog? Like a flower, but not a flower, what is foggy and foggy? I don't know myself, but it can only be "flowers are not flowers, and fog is not fog."

The sentence "Come at midnight and leave at dawn" seems to explain that the colorful thing turned out to be a dream! Dreamers are illusory. However, How Long Has It Been Like a Spring Dream denied that dream, but it was so similar to that empty dream that scenes of the past began to repeat themselves. That kind of beauty and happiness, that kind of memory, in the end, are fleeting, without a trace, leaving no trace.

At this time, Bai Juyi had already burst into tears unconsciously, and then looked up. It was almost dawn and the morning glow came into view. Although the morning glow is beautiful, it is easy to die and disillusion, adding a touch of sadness to Bai Juyi's heart. Even if the past is so short and hurried, the traces left in my heart are indelible, leaving only a little memory.

No matter what is left is happiness, happiness, pain and sadness, it is an inseparable yearning for his present life, which makes him always fall into heavy yearning inadvertently, because it is his irreplaceable past and memory after all. "Flowers are not flowers" leaves us with such a vague impression that we also feel lost after reading it, which may be the unique significance of this poem.