Keep in mind that Harper Lee published this novel in 1960 just as the Civil Rights Movement was beginning to heat up. I know that you probably already have a decent idea of what the Civil Rights Movement was, but I doubt that you have ever seen it quite like this.
The Little Rock Nine represents the intensity of racial turmoil occurring around Harper Lee.
The Emmitt Till murder is also significant to the story of To Kill a Mockingbird because it is a demonstration of what justice and the court systems looked like in the 1950's south.
*Warning- there are a few graphic images in this video. If you do not want to see his corpse, look away when they are discussing his funeral.
While the Scottsboro Boys happened long before Harper Lee was probably thinking about writing the novel, the story of these young men gives an insight into what 1930's Alabama (which is when the novel is set) and its treatment of African Americans looked like.