Search Engine Optimization (SEO) includes all measures to organically increase the presence on search engines (e.g. Google). The hit lists (SERP) are defined by the search engine by an algorithm and not by paying for presence, which is again the case with SEA.
The placement in the hit lists thus depends solely on the search terms entered by the searcher and the available content on possible web pages.
SEO thus aims to improve the organic traffic of a website and reach the right users.