In chapter 4 of To Kill a Mockingbird, you will be able to find the answers to your questions. In this chapter we see that Scout is sill struggling with her teacher and school. She longs for days of summer. While on the way home from school one day, Scout sees something shiny in the knot-hole in the tree. She finds two pieces of gum, and later she and Jem find more gifts left for them. The children are saddened when Nathan Radley cements the hole in the tree, stopping the small communication that they did have with Boo.
When summer finally comes, it brings Dill with it. Scout is happy to have Dill back, but he and Jem soon begin their campaign to bring Boo out of the house. Atticus had warned the children to leave the family alone, but Jem disobeys him and continues with his agenda. Scout is thinking of what she heard when she rolled in the tire into the yard of the Radley house. It has been bothering her for some time.
Through all the head-shaking, quelling of nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the side walk. Someone inside the house was laughing.
This chapter is important because it is the first time that Jem and Scout realize that Boo is trying to communicate with them. With Scout hearing the laughter coming from the house, this means that Boo is watching them, as well. Jem and Scout don't know it yet, but they are extremely lucky to have a person like Boo watching out for them.