SEO is the abbreviation for Search Engine Optimization, which would be defined by the process of increasing the visitors of a site, due to displaying as close to the top of the search engine results list (Search Engine).
It is already well known that most search engine users generally look at the first 3 pages of results. Specifically, the closer a site is to the first returned result (ideally it would be the first one), the more likely it is to be visited by users who initiated the search.
We can thus name the rank of a site as its order number in the list of results returned by the search engine. It is so easy to understand that it is very important for a site to have a lower rank to get more visitors through their search engines.
SEO helps sites to be "as good as" search engines. It helps them to understand more easily what is the site in question or what are the keywords that define the site. These things come to the aid of search engines, but, the main interest of SEO is a consequence of the ones listed above. Assuming we have two sites that deal with the same topic, and which have the same content, but, of which only one is optimized for search engines. When a user generates a search for one of the keywords that define the two sites, the one that has been optimized has a higher chance of appearing before the second one in the results list. This is also due to the fact that the search engine "better" understood what the optimized site is doing.
Going into more detail, there are two ways to approach search engine optimization problems. There is Black Hat SEO, and White Hat SEO. These two techniques are somewhat different from each other in their morality. White Hat SEO generally deals with those optimizations that help the search engine "better" understand the content of the site, while Black Hat SEO uses certain "getaways" of search alogrithms to bring its site. optimized page among the first results. While the first technique, White Hat SEO is a moral optimization technique, and has the best long-term effects, those who use Black Hat SEO are considered to be Spammers, and ... in general, the effects of their work can be seen in -a shorter period, but at the same time their long-term effect is to mark the respective site as a spam site, and to exclude it from the search engine results. It is obvious which optimization technique should be used for a site. However, there is much debate on the topic: "Which is more useful: Black Hat SEO or White HAT SEO?". There are people (generally those who use White Hat techniques) who say that Black Hat SEO should never be used, and that it is immoral and unnecessary to the general public. At the same time, Black Hat SEO followers argue that in general all optimization techniques are not moral, including White Hat, because these techniques are trying to "trick" the search engine. It was interesting to me the discussion between Earl Gray (Black SEO supporter) and randfish (White Hat) regarding the morality of the two optimization techniques.
In conclusion, SEO is a technique of optimizing the pages of a site (or the entire site) so that the content of these pages is as "within the meaning" of the search engines. This is not a definite one, but it is a brief presentation of the idea of SEO.