In Chapter 22, Huck witnesses a drunk named Boggs making trouble and cursing a man named Sherburn. This goes on for a while, and then Sherburn comes out of his store and gives Boggs an ultimatum: stop before one o'clock, or Sherburn will come after him. Boggs does not stop, and Sherburn kills him in the street. After the death, the townsfolk -- who gathered to watch the spectacle -- whip themselves into a mob frenzy and try to storm Sherburn's house to lynch him. They are met by Sherburn himself, standing with a gun, and he gives them a long speech that dissipates the crowd's enthusiasm:
"You didn't want to come. The average man don't like trouble and danger. YOU don't like trouble and danger. But if only HALF a man -- like Buck Harkness, there -- shouts 'Lynch him! lynch him!' you're afraid to back down -- afraid you'll be found out to be what you are -- COWARDS -- and so you raise a yell..."
(Twain, The Adventures of Huckleberry Finn, gutenberg.org)
Sherburn's point is that mob mentality usually has no conscious thought or bravery behind it; each person in the mob is buoyed by the next person in line, and so nobody takes time to think for themselves. In addition, the mob is usually composed of cowards who would never try the mob action alone; only in a group are they "brave" enough to act. Sherburn faces the mob down alone, and his courage and refusal to give in causes them to back down.