Martin Luther King's civil rights campaign in the America of the 1950s and 1960s achieved something most people can only dream of; complete societal change. To do it, he had to endure terrible abuse from utterly evil people.
Here, he's showing his son that we never bow to terrorism or intimidation. He's calmly pulling the remnants of a burning cross out of his lawn. The Ku Klux Klan put it there. A few years later, he won and they lost.