Mark Twain lived in the United States in the late 1800s AD, and he wrote books for both children and grown-ups. Mark Twain (whose real name was Samuel Clemens) grew up white in the South when black people were still enslaved there, and a lot of his books are about slavery and freedom. Twain grew up well-off - his father was a lawyer and a judge - but his father died when Sam was 11 years old, and after that his family was poor. Sam had to leave school and work for a printer. During the Civil War, rather than fight for slavery, or fight against his friends, Sam left the South and lived in the West, in Nevada.
In Tom Sawyer, Mark Twain began by showing that rich people, parents, schoolteachers, and ministers didn't always know better than poor children. His next book, the Prince and the Pauper, suggested that poor and rich people were just the same, only in different clothes. In Huckleberry Finn, Mark Twain introduced one of literature's first major black characters (after Uncle Tom's Cabin). Twain also spoke out against European and American imperialism and colonization.
By looking at slavery and freedom, rich and poor, Mark Twain created an American way of writing and thinking that was different from British stories. In most British stories, like Dickens novels or Jane Austen novels, the heroes were born with "good blood" - meaning their parents and grandparents had been rich, even if they themselves were not. Mark Twain suggested that being good depended on your own behavior - not on how rich your parents were, or what color your skin was.