Atticus wants his children to understand that courage is not just physical. He wants them to appreciate mental courage too.
Jem and Scout are dissatisfied with their father because they think he is old and feeble and cannot do anything. They are impressed with he shoots the mad dog. Atticus wants them to learn that courage comes in all forms though. It is not just someone with a gun in his hand.
When Jem attacks Mrs. Dubose’s flowers and she asks him to read to her, Atticus sees a perfect opportunity to introduce the children to real courage. He knows he will have to act courageously during the trial, and his children will too.
When Mrs. Dubose dies, Atticus explains that she was addicted to morphine and wanted him to read to her so she could kick the habit before she died. This, he tells them, is real courage.
It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. (ch 11)
This is important because Atticus himself displays real courage in taking Tom Robinson’s case. This line to Jem echoes what he tells Scout when she asks if he is going to win.
"Simply because we were licked a hundred years before we started is no reason for us not to try to win," Atticus said. (ch 9)
It is no coincidence that these two conversations are so much alike. There are real parallels in the situations. In each case, real courage and not just physical courage is required. Atticus knows his children will have to learn that. Here, Scout is trying to explain that courage is not just fighting, but walking away from a fight. Courage is acting morally when it's hard to do.