The main conflict in Harper Lee's novel To Kill a Mockingbird is the clash between races that permeates the author's narrative. A story that takes place during the 1930's in the American South, To Kill a Mockingbird revolves around the racial tensions surrounding the trial of a black man accused of raping a white woman -- the most emotionally-charged of issues during the pre-Civil Rights era. The approach of Tom Robinson's trial, including the arrival at the town jail of a lynch mob that confronts the accused's attorney, Atticus Finch, the moral compass of this fictional town of Maycomb, the trial itself, and the trial's tragic aftermath provide the singular focus that holds Lee's novel together. Certainly, the Finch children's (together with their friend Dill) preoccupation with the mysterious figure of Boo Radley comprises an important subplot, but it is the racism that infects Maycomb that constitutes the novel's most compelling conflict.
The depth of the racism that characterizes Maycomb in To Kill a Mockingbird is not confined solely to the views of the town's white residents, especially the character of Bob Ewell. The Finch family's African American housekeeper, Calpurnia, exposes Jem and Scout to the hostility among some blacks towards whites when she takes the children to her church. Such hostility, Lee is careful to reveal, however, is confined to the character of Lula. Unlike the institutionalized racism of the region's whites, the racial hostility directed towards Jem and Scout is limited in scope, with the overwhelming majority of the church's congregants openly welcoming of this unique visit by the white children.
To reiterate, the main conflict in To Kill a Mockingbird is that dividing the races of Lee's fictional town. The story takes place in the Deep South when institutionalized segregation was the norm, and blacks survived at the pleasure of whites. The trial of Tom Robinson was but a microcosm of that dysfunctional social milieu.
There are several conflicts but typically we consider the main character and their journey when finding the "most important" conflict in a novel. In TKAM, this is Scout, and Scout's main conflict is Character v. Society as she is trying to cope with her childhood world changing as she realizes what her society is actually like.
This is seen through her interactions concerning Boo Radley and the trial of Tom Robinson. With Boo Radley, we see his myth and childlike ideas like the Boo Radley game dominate the early part of the novel. The children dare each other to touch the house and make up lies and repeat rumor and gossip about Boo. However, by the end, Scout realizes that Boo is nothing like the stories she has heard and told, but instead, is simply different and withdrawn from the society she finds out has quite an ugly side.
With the trial, Scout and Jem realize that justice isn't always served, and people are not always concerned with right and wrong like their father Atticus, but instead they see people allow their prejudices and ignorance influence their decisions.
All in all, these issues all come down to a conflict of character v. society.