The story of Colonel Sherburn reads like a story within a story. It has nothing to do with the overall plot of The Adventures of Huckleberry Finn. However, it does illustrate a larger topic that must have been on Mark Twain's mind.

Colonel Sherburn is a shopkeeper who is described as being about fifty-five years old, well-dressed, and with a proud look to him.

Colonel Sherburn calmly guns down the town drunk, Boggs. Boggs was certainly obnoxious and scares Huck at first. However, Boggs is ultimately harmless. Colonel Sherburn does not seem at all perturbed with shedding blood and kills Boggs as calmy as someone might kill an insect.

After killing Boggs, the crowd that gathers seems very sympathetic to the murder victim. When they gather to lynch Colonel Sherburn, the old shopkeeper makes a poignant speech. He calls them a pitiful mob. He points out the cowardice of men gathering together to do something that they would each be afraid to do on their own in broad daylight. This cowardly mob, Sherburn points out, will only do ugly deeds within the anonymity of the crowd and in the dark. He sees all of them as cowards who would never make a stand if it meant any personal risk to themselves. This is the aspect of human nature that he cannot stand and he lets his would-be lynch mob know it. He tells them that the whole notion of southern bravery is a joke. Furthermore, this is a mob without a man at its head to represent them. That just makes them all the more cowardly to Colonel Sherburn as they represent nothing.

In Chapter 21 of Huckleberry Finn, Colonel Sherburn is a wealthy shop owner in Arkansas who Boggs, a drunk, is taunting. Boggs claims that Sherburn has swindled him. Sherburn tells Boggs that if he continues to taunt him, Sherburn will kill him. This is what Sherburn does, even though Boggs is with his daughter at the time. 

After Boggs dies, an excitable mob develops on the street and decides to lynch Sherburn. He stands before the crowd in a calm manner and addresses them about the cowardice of human kind. He says:

"Why, a man's safe in the hands of ten thousand of your kind—as long as it's daytime and you're not behind him."

He speaks of southerners who acquit a man when he's tried in court and then lynch him at night so that no harm will come to them. He ends his speech by saying,

"If any real lynching's going to be done it will be done in the dark, Southern fashion; and when they come they'll bring their masks, and fetch a man along."

In other words, people will only carry out murder when there is no chance that any harm will come to them. Sherburn's speech on the nature of human cowardice quiets the crowd, and they leave quickly, as does Huck.  

In Mark Twain's The Adventures of Huckleberry Finn, Colonel Sherburn is a store owner who guns down a local drunk named Boggs. Boggs has ridden into town threatening to kill the Colonel and insults and taunts him until Sherburn appears on the street.  After Sherburn kills Boggs, a number of the town's men decide to confront the store owner, leading to one of the novel's seminal events, an event that influences further developments.  

Consequently, in Chapter 22, the lynch mob arrives at Colonel Sherburn's home expecting to take matters into its own hands.  Sherburn steps out onto his porch to confront the angry mob and instantly takes command of the situation, declaring that they haven't got the courage or the "pluck" to take such a drastic action as lynching a man even though they can, in cowardly fashion, "tar and feather" prostitutes who venture into town; he declares he is safe around them as long as it's daylight and they are not behind him (marking them as the ultimate cowards):

“The idea of you lynching anybody! It’s amusing. The idea of you
thinking you had pluck enough to lynch a man! Because you’re brave enough to tar and feather poor friendless cast-out women that come along here, did that make you think you had grit enough to lay your hands on a man? Why, a man’s safe in the hands of ten thousand of your kind—as long as it’s daytime and you’re not behind him."

Labeling the mob a bunch of cowards in this way, Sherburn continues to lecture the group on its inability to stand on its own and to exercise any degree of courage in the face of true adversity.  The anonymity of a mob provides each individual courage he doesn't possess on his own.  As Twain has Sherburn, this rare figure of courage and dignity, state, a mob is the most pitiable group of humanity there is: 

"The pitifulest thing out is a mob; that’s what an army is—a mob; they don’t fight with courage that’s born in them, but with courage that’s borrowed from their mass, and from their officers."

Huck and Jim's journey has been filled with examples of man at his vindictive worst.  In Colonel Sherburn, they view a man of stature and foreboding who will not back down in the face of blustering, mobbing rage and cowardice.

Colonel Sherburn is a store-owner and the richest man in town. He is insulted by a drunk man named Boggs in the "Arkansaw" chapter (Ch. 22).  In regards to the mob speech, Sherburn describes humanity as being cowardly because of the mob-mentality it has.  The lynching mob is seen as sheep-like because it has a collective one-track mind: people see an opportunity to take justice into their own hands but never really question the motive or reason for doing so.  A wave metaphor is used to parallel this concept as a wave goes crashing in one direction without any idea of doing anything else.  Lastly, the crowd is easily swayed by Sherburn's speech on cowardice and it shamefacedly disperses.