When Jem complains that Mrs. Dubose says horrible things to them, Atticus tells him not to let it get to him.
Atticus gives Jem the following advice when he tells Atticus what Mrs. Dubose said about him.
“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 wants both of his children to learn how to avoid a fight. Mrs. Dubose insults the children by saying they run wild and complaining about Atticus defending a black man. He stresses the fact that it is there job to react appropriately when people say things to them. Mrs. Dubose is old and sick, and they should humor her to a certain extent. He is basically telling Jem to be the better man.
Jem does not take his father's advice. He attacks Mrs. Dubose's flowers in an act of rage, shocking Scout. Atticus is surprised that it was Jem and not Scout who lost his head.