Search Engine Optimization (SEO) is a way of designing websites to make them search engine friendly, so that the search engines can interpret the content and index it, in the database, accordingly for the users. The better the SEO strategies used, the more the chances of indexation and good ranks on the first SERP.
Be careful the internal SEO criteria that Google expects from a site.
The first thing to be done about SEO is primarily internal SEO. Internal SEO is done by organzing the page hierarchy well, connecting the pages in an order, defining and introducing the webpage for the search engines, telling what are those pages about and what is the porpose of the page in the first place.