The river is general, and the well is black. The yellow dog is white and the white dog is swollen. What does this poem mean?

This is a limerick. During Zheng De's reign in the Ming Dynasty, there was a surname in Wancheng, Third Brother, who inherited the skill of squeezing and made a living by selling oil with high quality and low price. For a long time, it was called "Zhang Dayou", and his brother liked to write poems. He once wrote "Ode to Snow" and said: "There are mountains and rivers, and there are holes in the wellhead. The yellow dog is white and the white dog is swollen. " Later, people called vulgar poems limerick. It's snowing. The yellow dog has snow. White, of course. Because of the cold weather, the white dog's skin is swollen with cold. So it's swollen.