Who knows the meaning of the sentence "the cold current brings the moon as bright as a mirror, and the night wind blows the frost as a knife"

The Seven Laws "A Farewell to the Jiang Lou" was written by him for Sima Shi in Jiangzhou. These two sentences mean that the moon shines in the sky and the cold current on the river is like a mirror; The evening breeze blows, and the frost is like a knife. Write the scene of late autumn and early winter, revealing the poet's sad mood in sadness and indifference.