In this scene, Scout is able to see everything as Boo had seen it for the past few years. She is finally able to fully understand Boo's perspective, and look at the world that she had been living in through his eyes. Lee writes a very touching sequence of what Boo must have seen as he sat inside his house, looking out the window. From his porch, she can see their house, Miss Maudie's house, and she realizes that Boo must have been able to see much of what had happened in her life in the past several years. He had been as much a part of her life as anyone else, and as it turns out, a true friend.
The sequence highlights that we really don't know people at all until we have walked in their shoes and seen things from their perspectives. Boo saw much more than they had realized; they just thought he was only a creepy guy who had stayed holed away from society. In fact, he had been a part of their lives, living vicariously through them, and because of his constant presence, he had been able to save them. It ties the threads of everything that had happened to Scout and Jem together, along with a major theme of the book, which is to not judge people on appearances or rumor, but on first-hand experience of their lives. I hope that helped a bit; good luck!