Here our speaker starts by talking to the road. It seems like a strange conversation partner, but our speaker tells the road that it's not the only thing around.
He suspects that there are a lot of unseen things around as well. Okay…
Then the speaker identifies in this "the profound lesson of reception" (17). In other words, there is deep insight to be gained here about the nature of perception. ("Reception" is best understood as referring to how the speaker receives the world, how he takes it all in.)
That lesson is—wait for it—"neither preference or denial" (17). In other words, our speaker is taking in every part of the world, without exceptions or preferences. His view is taking all comers.
As a result, our speaker takes in all kinds of people, from African Americans to folks who are diseased or illiterate.
If that strikes you as a bit offensive, then you're paying attention. Today, we wouldn't think of equating African Americans with diseased or illiterate people. However, Whitman was writing this around the time of the Civil War, when slavery was still a reality. Check out our "In a Nutshell" for more on that.
The point here is that the speaker recognizes people from every walk of life, even the most marginalized parts of society.
He also notes an assortment of other things, like a "drunkard's stagger," a "laughing party of mechanics," a "rich person's carriage," even "the moving of furniture into the town" (19-21).
Like all these things, the speaker describes how he travels down the road ("I also pass") (22). Nothing can be "interdicted," or kept off this path (22).
At the same time, the speaker accepts and holds everything dear—what a guy.