What do Grain Rain's "three stages" mean?

The first one is waiting for Ping to be born, the second one is waiting for pigeons to brush their feathers, and the third one is waiting for Dai Sheng to drop mulberry.

In ancient China, Grain Rain was divided into three stages: the first stage was equal birth, the second stage was waiting for the cuckoo to brush its feathers, and the third stage was waiting for Dai Sheng to drop mulberry. After Grain Rain, rainfall increased, duckweed began to grow, then cuckoo began to remind people to sow, and then Dai Sheng birds began to be seen on mulberry trees.

After the solar term in Grain Rain, rainfall increased, and rain gave birth to hundreds of valleys. With sufficient and timely rainfall, cereal crops thrive. In the south where the grain is abundant, poplars and willows fall, cuckoos cry at night, peonies bloom, cherries are red and ripe.