Introductory things are always the first step before more complex material. You must pass the introductory exam before tackling advanced topics.
Pro Tip 2/3
Not Complete
An introductory section won't give you all the details, just a starting point. Read the introductory chapter to get a general idea of the book's theme.
Pro Tip 3/3
Broad Overview
Introductory material provides a broad understanding, not deep knowledge. The introductory lecture gives a broad overview of the subject's history.