What is Search Engine Optimization (SEO)?
First you need to know what is Search Engine.
Is a tool used to search for information on the Internet, once you enter the search key like a word showing your search results from more than one website.
Search results are displayed on many pages. Example:
Google is the most popular search engine, when you search about something you find on the first page 10 results.
Have you ever asked yourself why this site exists in # 1 and other is # 5 in the result, or in the second page in result of your search for example?
The answer in SEO, is the control. SEO it configures site for search engines and makes your site more compatible with all search engines by doing some actions on your site, to reach the results at the first pages in the results of different search engines.