Explain about Jem's outburst when Scout asked him about Miss Gates hating Hitler. What did Atticus say to her? What did he mean?

Jem links any prejudice with the trial: they are painful memories. Jem responds very angrily, and tells her he never wants to talk about anything having to do with that trial again. Scout is taken aback and goes to Atticus, who assures her that Jem just needs some time to think about things, and then he'll be himself again.