Every site is indexed on Google. This is an automatic process that Google performs in order to then put it forward as an answer to the questions asked by web users.
Ranking on Google, although it happens automatically, is subject to the contents of the site, and a site with good SEO is more likely to rank better on Google. It is often thought that SEO is just a way of using language, or the use of certain words. This may be true, but it is not entirely sufficient for marketing and positioning purposes. SEO is the combination of operations, words, links, formulas and much more that only a trained SEO agency can offer you.