In chapters 21 and 22
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If you read the speech carefully you will see that it is fairly pointed. Twain is suggesting that true courage is to do the right thing, but most men will not stand up for the right thing. In fact, "Southern justice" as Twain points out, is often committed by a gathering of cowards who hide their faces and become judge, jury and executioner. Sherburn can stand up to the multitude because no one in the crowd has the courage to defy him. Courage is standing in the face of evil and defeating it, but Sherburn points out that no one is willing to do that.
Expand the idea and Twain is talking about slavery and the treatment of blacks. He is suggesting that Huck's actions, although slow in evolving, suggest that courage can be found if we are willing to defy social norms or great crowds and do what our "deformed conscience" tells us is right. That is why when Huck stands up to the program that society has laid out for the treatment of slaves, and says that he would "go to hell" rather than see Jim returned to slavery, it is in keeping of the colonel's definition of courage. This excerpt was taken from the source link below. Please take a look.