Jem finally recognizes that it may be Boo's own choice to stay inside his house. With an outside world that can so blindly convict an innocent man like Tom Robinson, why join them? Why should Boo attempt to integrate himself into a community that spews unfounded gossip about him? By isolating himself inside his family home, Boo may deliberately decide not to involve himself with people on the outside who treat others badly or unfairly. Inside the Radley house, Boo is immune to such influences and can maintain his own innocence.
"Scout, I think I"m beginning to understand why Boo Radley's stayed shut up in the house all this time...it's because he wants to stay inside." (Jem, Chapter 23)
Jem is so angry with the outcome of the trial he almost cannot see straight. As a child, he doesn't understand that if he can see Tom Robinson's innocence, why can't everyone else? In the chapters following the verdict, Jem begins to understand what prejudice means to the town of Maycomb, and why Tom Robinson never had a chance.
He reveals his understanding of this with the above quote. Suddenly - Boo Radley is no longer strange, or if he is, Jem would rather be on his side than the side of everyone else.
boo wants to maintain his own innocence and stay hidden inside away from the gossip and drama.
JEM NOTICES THAT IT WAS BOO RADLEY'S CHOICE TO STAY HIDDEN FROM THE REST