What does Duke Zhou mean by dreaming of so many snakes in a sack?

Snakes are a symbol of trouble and entanglement. There are so many snakes in the dream, which shows that the dreamer has been depressed recently.

Your background and identity are unknown, so you can't infer what your troubles are. But the yellow snake is either talking about economic issues or talking about sexual issues.

If you mean economy and money, it means that your recent economic troubles have been slightly relieved and rich; If it refers to sex, it means that the suitor gave in.