Atticus has a conversation with Scout regarding the upcoming trial and his defense of Tom Robinson, who is an innocent Black man wrongly accused of raping a white woman. Scout asks her father why he is defending Tom, and he tries to explain to her that he is morally obligated to protect him. As a proponent of equality, Atticus is determined to prove Tom's innocence but understands the odds are not in his favor.
Scout then asks Atticus if he will win the case and, he responds truthfully by saying, "No, honey." Scout follows up by asking why he would even bother defending Tom if he doesn't have a chance of winning and Atticus says,
Simply because we were licked a hundred years before we started is no reason for us not to try to win.
Atticus means that simply because the odds are heavily against him, there is never an excuse not to follow one's conscience and do the right thing. He is well aware of Maycomb's systemic racism but is determined to stand up for equality and justice at all costs.
Atticus is teaching Scout a lesson on "real courage," which he later emphasizes when his children interact with Mrs. Dubose. He is attempting to instill courage in Scout by teaching her the value of following her conscience in the face of adversity. Defending a Black man in a segregated society founded on Jim Crow laws is no easy task. However, Atticus is the only person willing to fight the uphill battle and behaves like a consummate role model for his children. His brief conversation with Scout regarding the upcoming trial teaches her about "real courage," the importance of following one's conscience, and doing the right thing in the face of adversity.