The title of this poem, "Where the Sidewalk Ends," is also the title of the entire book in which it was published, so we should definitely pay it some attention.
What's so amazing about this title is that it manages to represent two things at once. For one, it's talking about a fantastical place, a land beyond the city where the sidewalk literally stops. It's a magical place full of crazy things like soft white grass, moon-birds, and peppermint wind.
However, like most fantasylands, the place where the sidewalk ends is also a metaphor. It represents the world of the imagination, a place we all can travel to no matter when or where – whether we're in a city full of sidewalks or sitting in the middle of a field.
This title stands for an important idea for children and adults alike: no wonder it's the title of not just this little poem, but an entire book. It sums up what a lot of Silverstein's writing does to his readers: takes them to the place where the sidewalk ends, into the inner workings of their own creative minds.