This couplet is from the Caotang couplet on the Lake in Yangzhou, which was inscribed by Yi Bingshou, a famous calligrapher in Qianlong period of Qing Dynasty.
Shanglian Baiyun Chuqing old rain is smooth.
"Baiyun Chuqing" refers to Chuqing after the rain, and the weather is fine (you can see blue sky and white clouds);
"Old Rain" refers to an old friend.
"Fit" means: just right, just right.
"arrive": arrive
Therefore, the complete meaning of the first couplet is: after the rain, Chu Qing, blue sky and white clouds, at this time, old friends have just arrived;
The bottom line has not been paid, and Gao Tan's turning to Qing Dynasty has nothing to do with Ye Shi, a poet in the Song Dynasty. Instead, it directly quoted a famous sentence from Li Bai's famous parallel prose "Preface to the Spring Banquet in Taoliyuan" in the Tang Dynasty. Please see the following excerpts from the original text:
. . .
Qun Ji Jun Xiu, all for Huilian;
We sing, and we are ashamed of our happiness.
You didn't get a reward, you can tell me. (Note: In ancient Chinese, "Tan" and "Tan" are the same word)
Give a banquet, sit on flowers, fly feathers and get drunk on the moon.
. . .
Li Bai's book "You Shang" is not finished yet. Gao Tan turns to Qing, which means: elegant enjoyment, strong interest (never stopped), and clear words and elegant language. (Qing here refers to the kind of "speaking freely" popular in the circle of scholar-officials in Wei and Jin Dynasties-discussing debates on metaphysics and philosophy)
Yi Bingshou's second quotation is also an expression: When an old friend arrived, the two sides chatted happily. Tan Xing was very strong and kept talking.