What is "Shadow Shakes thousands of feet Dragon and Snake" about?

It means that the water image is like a dragon and snake swimming in the water, swaying at a height of thousands of feet, describing the huge wind and waves as a sign that it will set off a stormy wave.

From Gu Song written by Shi Yannian in Song Dynasty.

Original text:

Straight spirit is full of shame, iron clothes make purple scales dry. Shadow shakes thousands of feet Dragon and Snake, and the sound is stormy for a long time.

The moss is detached from the stone, and the green radish is attached to the cloud. Those who report the emperor's arrogance will regard it as a pillar of heaven in the class.

Translation:

Gu Song's straightforwardness, he is ashamed to bend the plate, but his words are empty. Gu Song flies like a snake, and its shadow shakes thousands of feet. Its momentum is like a storm, shaking the air and chilling. It's really thrilling! The humble position of moss and bamboo attached to the climbing shows the integrity of Gu Song, and appeals to those in power to use the superior materials buried in the mountains as the top beams and pillars for building the main hall.

Extended data:

Shi Yannian (994 ~ 104 1) was an official, writer and calligrapher in the Northern Song Dynasty. The word Man Qing, the word An Ren. Originally from Youzhou (present-day Beijing area), Youzhou was ceded to Qidan in the later Jin Dynasty, and its ancestors moved south and settled in Song Cheng (present-day Shangqiu South, Henan Province).

After repeated attempts, during the reign of Emperor Zhenzong, the right classroom was straight, the Taichang Hall was changed to Taizhu, the Dali Temple was moved, and the official went to the Secret Pavilion to manage the school, and the prince was allowed. During this period, he wrote the poem "Gu Song", calling on those in power to use the fine materials buried in the mountains as the top beam and pillar of the main hall.

Shi Jie, a writer in the Northern Song Dynasty, is also known as the "Three Masters" with Shi Yannian's poems, Ouyang Xiu's essays and Du Mo's songs.

Baidu Encyclopedia-Shi Yannian