Sherburn shows a deep and abiding disdain for the mob that assembles with the intention of lynching him. He disperses the mob as much with his low opinion of them as he does with his shotgun.
The town threatens to lynch him, but his scornful speech about the cowardice of the average American man and the mobs he participates in breaks up the crowd.
In this speech, Sherburn associates the mob with the "average man", presenting a general commentary on the moral qualities of the people of this culture. The average man is not brave. The average man is a coward, Sherburn says.
"Your newspapers call you a brave people so much that you think you arebraver than any other people - whereas you're just asbrave, and no braver."
Sherburn comments on the cowardice of juries in this culture, saying that they are not brave enough to convict murderers. He goes on to say that the crowd only brought with them one "half-man", Buck Harkness, and that half-man would not be enough to go through with the lynching of a true man. The mob's response is to yield entirely to Sherburn's attitude, breaking up and going away.
The specific subject of the commentary made by Sherburn has very little to do, directly, with the action of the novel.
Twain speaks out against lynch mobs who do not fight with courage but come like cowards in the middle of the night wearing masks.
The novel, on the whole, is not interested in lynching and the weakness of a legal system populated by people unable to act honorably or courageously. However, Twain's use of the Sherburn episode can be interpreted as a commentary on integrity in American culture - a theme that is central to the novel.
The King and the Duke, along with many other examples, constitute a significant exploration of fraud, hypocrisy and moral weakness as it exists along the Mississippi in this era. Huck Finn is challenged himself to act courageously, going outside of his culture's moral stipulations to do the right thing, without his community's support or agreement.
Being morally brave and acting with integrity, for Huck, ultimately means deciding for himself what is right.