SEO is the strategy of optimizing a web page for search queries. Search engines discover content on the Internet and then rank it by relevancy to topics. When somebody searches for an answer, the results he finds in the engine are called organic, and if he clicks and visits a page, this traffic is called organic, natural or free. Today, depending on the search query, you may encounter different results formats like videos, image carousels, video carousels, local pack (map), Ask boxes. Many pages are in the Top 10 results because they have worked one way (SEO) or the other (rankings manipulation).
Note that organic traffic exceeds all other traffic channels like social media, paid ads, yet it costs less money than those, has lasted effects over time (retention) and attracts more clicks (paid ads in the USA attract less than 3%).