Atticus wants his children to behave politely to Mrs. Dubose, no matter how she insults them.
Mrs. Dubose is a mean lady described as “plain hell” by Scout. She shouts insults at the Finch children as they pass by, even before their father defends Tom Robinson. Once the trial commotion starts, the insults become racial and directed at their father. This annoys Jem.
As with Scout, Atticus’s advice to Jem is to not let things get to his head. He wants Jem to behave, even if she doesn’t.
"Easy does it, son," Atticus would say. "She's an old lady and she's ill. You just hold your head high and be a gentleman. Whatever she says to you, it's your job not to let her make you mad." (ch 11)
Atticus thought that Scout would be the one he would have trouble with, not Jem. Although Jem is older, he gets angry enough at Mrs. Dubose that he snaps. He destroys her flowers, and Atticus makes him apologize and spend time reading to Mrs. Dubose. He later explains that she was trying to kick her morphine habit, and she was the bravest person he knew. He wanted Jem to see true bravery.