This quotation is part of Scout's narrative from the final chapter of the novel as she looks out over her neighborhood from the Radley's front porch. (Scout does not actually say this aloud; it is only part of her narration.) After walking Boo Radley back to his house following the attack on the children by Bob Ewell on Halloween night, Scout pauses to let her father's advice sink in, to stand in Boo's "shoes and walk around in them." Walking in them was not necessary for Scout:
Just standing on the Radley porch was enough. (Chapter 31)
As she stands on the Radley porch, it is as if she is seeing the past years' events through Boo's eyes as he would have seen them from the window. It is a whole new perspective for Scout, and she now understands that Boo must have been keeping an eye on "his children" for many years. The quote refers to that very Halloween night, an autumn evening when Boo must have recognized that the children's safety was in question, and he decided to emerge from his self-imposed exile to rescue them from the murderous hands of Bob Ewell.