Chapter 14 is a coming of age chapter for both Jem and Scout. More so for Jem than Scout. Jem is becoming older, more mature, and more socially conscious. That is why he feels the need to lecture Scout on how to not act like a child. Of course Scout doesn't react super well to that, because she IS a child still.
Chapter 14 is also the chapter in which Dill returns to Maycomb. He has run away from his home. This indicates further to the reader at how good a father Atticus is and how good he is at keeping a household. It also shows that Aunt Alexandra, while a nag, acts out of love. Dill likes all of that.
Jem does go tell Atticus that Dill is there and hiding, and Scout feels like it is a betrayal. It isn't. It's Jem taking another step into adulthood by doing what is right and not what he feels like doing.
As Chapter 14 of Harper Lee's novel To Kill a Mockingbird begins, Atticus Finch is under considerable pressure from two sides. Aunt Alexandra is pushing hard for a change in how he raises his children, especially Scout, the young narrator of the story and a girl prone to what was then considered "boyish" behavior. On the other side, some of Maycomb County's less-enlightened residents are increasingly vocal in their condemnation of Atticus for agreeing to defend an African American man, Tom Robinson, against the charge of raping a white woman--an extremely emotional issue in the American South in the period depicted in the novel. Alexandra has been arguing for a more appropriate method of raising Scout, one more suitable to her feminine gender. Atticus is resisting, but the reader gets the impression that he is also tiring. In Chapter 14, Lee has Scout and Jem continue their emotional journey towards eventual adulthood.
A key element of Chapter 14 in To Kill a Mockingbird involves the nature of the crime of which Tom Robinson has been accused: rape. In response to Scout's innocent questions regarding the definition of that particular crime, Atticus can only muster a very legalistic definition: "He sighed, and said that rape was carnal knowledge of a female by force and without consent." It is clear to Atticus that his children are maturing and that the time has come to discuss the "facts of life." While this issue is being addressed, however, Lee includes in this chapter a climactic confrontation between Scout and Aunt Alexandra, the former tired of the latter's intrusions into her life, the latter determined to influence the direction in which Scout is heading. The result is a heated confrontation between Atticus and Alexandra regarding Atticus's parenting skills. So heated does the situation become that Scout even becomes intensely angry with Jem, who is trying to calm her down, his greater period of time on the planet lending him more wisdom than his still-young and emotionally immature sister.
Chapter 14 also illuminates the sadness of Dill's home situation and contrasts it with the relative stability of that of Jem and Scout. Dill has run away from home and made his way to the Finch home. Jem and Scout discover that Dill is hiding under Scout's bed. When questioned, Dill provides a disturbing but clearly misleading story involving "having been bound in chains and left to die in the basement." Conjuring up tales of an abusive stepfather, Dill proceeds to describe an exceedingly bizarre journey to the Finch home. Once Atticus enters the situation, the point of this part of Chapter 14 becomes clear. Dill's story is, of course, entirely fabricated. What he observes in the Finch home, however, is a tight-knit family, each member interacting with the others and clearly emotionally involved in each other's lives. As Dill explains his decision to run away from home, he says of his mother and stepfather, "they just wasn't interested in me."
Chapter 14 is an important chapter in To Kill a Mockingbird because of its emphasis on the emotional maturation of the Finch children and because it provides an opportunity for Scout in particular to learn to better appreciate what she has.