How will people find your website when the world-wide web is such a huge place? I think you know the answer:
They will Google it!
And that's where SEO comes in, search engine optimisation. SEO is a huge area, but this book focuses on on-page SEO - the things you can do on your own website to make it rank better in Google.
On-page SEO is about the content that you publish on your website. It is about optimising that content for both search engines and readers. This is an important skill if you want your website to thrive. It is also a lot of fun. Good luck with your on-page SEO!