What is the meaning of "Yu" in ancient poetry?

"Stop and sit in the maple forest late, and the frost leaves are red in February." This poem is taken from the famous poem "Mountain Walk" written by Du Mu in the (Tang) Dynasty.

Stop and sit in the maple forest late, and the frost leaves are red in February flowers.

Tang Du Mu's "Mountain Walk"

I stopped the car because I like to enjoy the night view of Fenglin. Frosted maple leaves are even hotter than flowers in February.

[Appreciation] This poem is about autumn scenery seen in the mountains, and the last two sentences are particularly beautiful. Therefore, Huang Sheng's "Don't Find Gold in Tang Poetry" says that "there is a picture in the poem" and it is a "travel map of autumn mountains". The word "sit" in the poem is interpreted as solving, not sitting down. "Frost leaves are redder than February flowers" has been praised as a "true sentence" by predecessors, because it fulfilled the previous meaning and showed us a maple forest in autumn. You see, under the autumn sun, the maple leaves are about to drip, and the layers of forest are all dyed, which is more beautiful than the spring flowers in February! Through this piece of red, people see that autumn has the charm of spring. Can you not feel relaxed and happy and linger?

[Original] There are people in Bai Yunsheng, far away from the cold mountains and with inclined stone paths. Stop and sit in the maple forest late, and the frost leaves are red in February flowers.