Perhaps the most important thing to understand as you read To Kill a Mockingbird by Harper Lee is its setting and context. The book was published in 1960, before the passage of the Civil Rights Act and is set in Alabama where racial discrimination and even lynchings were still common. While in the twenty-first century it would not be unusual for a white lawyer to defend a black defendant, for Atticus Finch to do this was very daring and radical, risking his own life and that of his children for the sake of justice. The plot reflects the way in which brave lawyers were an essential part of the civil rights movement.
The next important thing to understand is that the protagonist of the novel is Scout, and the racial issues and Atticus' stance about them show how a parent's moral choices influence the way a child matures. Thus, it is important as you read, to see how Scout develops as a person and a moral agent.