They "debate" Listwell's occupation and purpose, even though it is none of their business, and then they settle down to gossip and drink, not really doing anything to help solve problems or find answers to questions like slavery. They are like the people of the nation, but they are like the Congress as well, because the Congress often debates issues to death, but never really does anything to solve them. In particular, they represent the issue of slavery, because Congress and those who created Congress debated the issue too, but never managed to come up with a workable or viable solution to ending slavery. Thus, the tavern represents the nation and the people inside represent the lawmakers, who are not doing their jobs. Finally, the tavern, and its non-descript and decrepit outbuildings represent the nation in another way. The outbuildings, like the tavern, are falling apart, and many of them have already collapsed from disuse and disrepair. The tavern is on its way to join them, too. Douglass is saying that the nation itself suffers from...

Of course, he was right, because the issue of slavery eventually helped lead to the Civil War between the North and South, which create animosity and misunderstanding on both sides, but did lead to the end of slavery in the nation. Obviously, Douglass felt the nation was on shaky ground when he wrote this short novel in 1852, and he was not wrong. The tavern represents everything wrong with the institution of slavery and what will happen if it is not abolished, while Washington represents the good, decent person who should have been allowed to voice his opinion and stop slavery like the Founding Fathers should have done.
References

Douglass, Frederick. "The Heroic Slave." University of Virginia. 2008. 14 April 2008. http://etext.virginia.edu/etcbin/toccer-new2?id=DouHero.sgm&images=images/modeng&data=/texts/english/modeng/parsed&tag=public&part=1&division=div1