Congratulations on making it through this book! HTTP is at the heart of the internet and therefore a good starting point for learning web development. Understanding HTTP and things like GET, POST, sessions, cookies and what it means to be "stateless" are vital. Understanding the basics of HTTP means you know what is happening "behind the scenes" when you build a web application. Learning about security means you're aware of threats like XSS, session hijacking and the countermeasures you need to guard against them. If you are learning HTTP for the first time, it's likely you'll have to go through some of the chapters more than once. Re-reading, reviewing, and exploring is normal when you are learning something new, and we encourage you to use other resources in your quest for greater understanding.
Although this book is by no means a comprehensive write-up on HTTP, we hope that the concepts covered in this book will give you a solid foundation on which to build your career as a web developer.