A chapter is a section of a book. Some long novels might be divided into twenty or thirty chapters.
The noun chapter is good for talking about a subset or small section of some larger thing. Book chapters are one example, and so is a chapter in a person's life or a chapter of an organization or club. When you mention the "mopey teenage chapter" of your life, you mean one small portion of the years you've lived. And the Montana chapter of a national fan club is made up of only members who live in one state.