Search Engine Optimization (SEO) is process of publishing the quality and unique content on the web pages to attract the users through search engines. SEO boots the unique content based on the user search keywords and search engines prioritize them to show up on their search engine result pages. This is one of the process where the content writers and website owners usually follow all the best practices to be on top of search engine results to attract the new users. Moz is one renowned company and providing services on search engine from longer time. They have created a 100% free course on SEO for beginners to learn SEO basics.
If you are a book lover, I would suggest this book to learn Fundamentals of Digital Marketing written by Puneet Singh Bhatia and published by Pearson to understand better.