What do these two sentences mean before the mulberry leaves fall? Where does this poem come from?

1, these two sentences mean: when the mulberry leaves are not falling, they are covered with green branches. Mulberry leaves have fallen, yellow and haggard.

When the mulberry leaves don't fall, their leaves are fresh and moist. Metaphor is a good time in life. When mulberry leaves fall, their leaves turn yellow and fall off one by one. Describe that married life is getting worse and worse.

3. "Meng" in The Book of Songs. This is a folk song, dating back more than 2700 years. With a woman's mouth, tell her emotional experience and profound experience frankly. It is a vivid portrayal of love scrolls, and it also left valuable information about local conditions and customs at that time for future generations. Today, it is still enlightening.