David has several dreams that are critical to the story line. However, I think that the question is asking about the dream that David describes in the opening paragraphs of the book. He dreams of a city that appears fantastic in every way possible. It is brighter, cleaner, and so on.

It was a beautiful, fascinating place, and once, when I was still young enough to know no better, I asked my eldest sister, Mary, where this lovely city could be.

The city is loaded with all kinds of fantastic technology as well. David is amazed that the city is full of carts that are not pulled by any animal of any kind. This is one concrete difference between Waknuk society and the city in the dream. He even dreams of amazing flying ships that are shaped like fish.

And sometimes there were things in the sky, shiny fish-shaped things that certainly were not birds.

The entire place just seems amazing beyond imagination. It seems open and free, and that is definitely a major departure from Waknuk society. Waknuk society is very controlled. Persecution of anything that is deemed not normal is a regular, everyday occurrence for the Waknuk people.

People in our district had a very sharp eye for the odd, or the unusual, so that even my left-handedness caused slight disapproval. So, at that time, and for some years afterwards, I did not mention it to anyone—indeed, I almost forgot about it, for as I grew older the dream came less frequently, and then very rarely.