What does it mean to classify the collected poems by year?

Different times and backgrounds have created different kinds of poems.

For example, the Tang Dynasty made the country rich and the people strong, with a strong poetic style. For example, Li Bai. In the Northern Song Dynasty, the country was poor and weak, and poems and songs made people eager to make contributions and serve the country. Like a land trip or something

That's what it means. You got it?