When Miss. Maudie's house catches on fire, the whole town comes out to help. Atticus tells Jem and Scout to stand in one spot and not to move. It is a freezing cold night and the children are frightened by what has happened. That winter is the coldest winter people can remember. They have actually had snow, which is pretty rare in that part of the country.
Jem and Scout are so mesmerized by what is going on, that they don't even notice that someone has put a blanket around Scout's shoulders. Atticus gets mad at them, because he thinks they disobeyed him and went back to the house to get a blanket. Jem and Scout promise they hadn't been anywhere and Scout can't explain how the blanket got around her. Jem realizes that Boo was the one who had put the blanket around Scout. This makes Jem realize that Boo was watching them, and showing concern for Scout, he had slipped out and placed a blanket around her. This makes Jem realize that Boo is not the scary person they all believed him to be.
In this chapter, we see more of the love that Boo has for Jem and Scout. He wants them to be safe. He was concerned for Scout. He is showing a real protectiveness over the children. We see that Jem is starting to realize that Boo is actually a good person and not someone to frightened of anymore.