When Scout asks Atticus if they are going to win the trial, he tells her they won’t. The tradition of racism is so strong in Maycomb that just the fact that a black man was accused of rape by a white woman is enough.
Scout compares the concept to the Civil War, but Atticus tries to explain.
This time we aren't fighting the Yankees, we're fighting our friends. But remember this, no matter how bitter things get, they're still our friends and this is still our home. (ch 9)
Atticus knows he is not going to win. He knows that he is generating hard feelings and stirring up trouble in Maycomb. Yet he continues to defend Tom Robinson, because it is the right thing to do. He tells Scout he could not hold his head up if he did not.
Atticus knows he has an important place in society. Even if some people do not like what he is doing, he is still well-respected in the town of Maycomb.
Atticus is also a remarkable man because he is doing what he thinks is right, and teaching his children about moral courage. He sets a good example for them to do what they need to do no matter how difficult it may be, and even if the result will not likely be what he wants. It's the effort that counts. He knows he is going to do his best, even if he can't win.
It means that just because 100 years ago it was different doesn't mean that they should just give up