Scout and Jem have begun to drift apart by this point in the novel. Dill and Jem have been off on their own playing and plotting while Scout spends time with Miss Maudie and Calpurnia.
When Scout learns about the plan to place a note on Boo Radley's window, she continues to hold her place outside of the small circle of Jem and Dill, encouraging them to drop the plan.
The previous posts are right about what happens in the chapter, but I think that the chapter (like most of the book) is about growing up. It is about Scout and Jem being treated more like adults and being expected to start to see the world more like adults -- in a more mature way.
In addition to the children intensifying their efforts to make contact with Boo, there is some background information about Miss Maudie. We learn she is a widow, that she inherited her gardening interest from her father, and that Jack Finch (Atticus' brother) has been jokingly asking her to marry him for years.
You can read a summary of chapter 5 here:
Much of chapter five centers around Boo Radley and what the children know of him. They try to find out more information, and they try to leave him a note with a fishing pole.
In to kill a mocking bird what is chapter 5 about?