Jem wants to forget how ugly people can be, something he witnessed during the trial. He tells Scout that he doesn't ever want to hear about the courthouse again. The trial was a very difficult and scarring experience for him and he is having a terrible time handling the reality of what he learned from it. Atticus tells Scout that Jem is trying to forget something but he was really storing it away until enough time passed. What is meant is that he is trying to forget about it until he is old enough to process it properly.