What's the meaning behind the poem "Where the Sidewalk Ends"?

"Where the Sidewalk Ends" is a poem about getting to a place outside of the city.  Stanza two narrates details about the negative parts of city life.  

Let us leave this place where the smoke blows black
And the dark street winds and bends.

The sidewalk is a symbolic roadway to getting out of the city.  When a person has reached the end of the sidewalk, he has reached the outside of the city.  What's outside of the city?  Great happiness, beautiful scenery, and a peaceful aura.  

And there the grass grows soft and white,
And there the sun burns crimson bright,
And there the moon-bird rests from his flight
To cool in the peppermint wind.

Now, whether or not Silverstein is writing about a real location outside of cities or an imaginary one is up for some debate.  Personally, I think it's an imaginary location.  If you have a copy of the book, the cover has a picture of where the sidewalk ends.  It's basically the edge of the known world.  I've linked a picture below.  I think Silverstein is reminding and encouraging readers to return to the joys of having a child-like imagination.  It's a form of escapism, and Silverstein wants to remind his readers how great those imaginary adventures were to those great places that had nothing to do with city streets, traffic, deadlines, etc.  I think the meaning of the poem is its encouragement to readers to remember to have some child-like fun and imagination.