Arthur Radley, whom the children referred to as "Boo", had emerged from his home for the first and only time in the novel when he realized the children were in danger, and killed Bob Ewell before Ewell could kill the kids--which he fully intended to do, having insinuated it within earshot of some townspeople in the weeks after the trial. The significance of this lies in Arthur Radley's knowledge of the children's impending danger--he knew they were in danger because he had been watching them from within his home, where he had been more or less imprisoned for years. He had laughed at their hijinks, repaired Jem's pants when Jem lost them in an ill-conceived adventure to look into the Radley home, left them little gifts in the hole in a tree (until his brother cruelly filled the hole with cement), covered Scout's shoulders with a blanket when she stood freezing on the street as Miss Maudie's house burned down--and finally, as the kids returned alone on that dark Halloween, he somehow knew they were being followed and was able to get to them, and Ewell, before Ewell could carry out his drunken, crazed plan. When Radley carries Jem into the house, no one is surprised to see him except Scout, when she figures out who he is. It is an interesting and somewhat sad climax to the even sadder story of Arthur Radley. In fact, Scout says as much when she reflects on the things Arthur had given them over the years:
Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good luck pennies and our lives. But neighbors give in return. We never put back into the tree what we took out of it; we had given him nothing, and it made me sad.
There is also some irony worth noting in Boo's saving the children, because when they first become aware of him, at the novel's beginning three years prior, they are terribly frightened of him and what they have heard about him. Thus, his returning Jem to safety at the novel's conclusion creates a kind of full-circle, and dramatic ending to the novel.