What does the ancient poem mean by "ping" after the Tang Dynasty?

Flat tone is the tone of words used in China's poems. "Flat" is straight, and "flat" is tortuous. According to the rhyming books revised from Sui Dynasty to Song Dynasty, such as Qieyun and Guang Yun, there are four tones in Middle Chinese, which are called Ping, Shang, Qu and Ru. Except for the flat tone, the other three tones have high and low changes, so they are collectively called Legato.

There is a certain format in the use of flat and even words in poetry, which is called meter. Pingsheng and Silence refer to the poetic rhythm composed of Pingsheng. Flat tone is an attempt to dualize the four tones. Four tones are four tones in ancient Chinese. Tone refers to the level, fluctuation and length of sound. On the basis of four tones, the flat tone is summarized through incomplete induction. Flat refers to straight, flat refers to twists and turns.

Up, down, in tune, the rest are flat.

I hope it helps you.