"Where the Sidewalk Ends" is all about the world of nature that lies beyond the borders of the city. You could also see this place as being even further away, in a supernatural world that can only exist in our imaginations. Of course, this imaginary world is in some way represented by images from nature. Moon-birds and peppermint wind might not actually be anything like the world of nature we're used to, but they are based on things we find in nature. The city represents, then, the world of the everyday and mundane that our imaginations help us to escape.
In this poem, nature represents the world of the imagination, while the city represents the mundane and everyday.
This poem suggests that happiness can only be found in nature.