SEO stand for "search engine optimization" is a process of improving your website with some techniques such as keyword implementation in webpages.It includes everything statring from the web page design, code to content in the webpage, it is basically optimize the website to make it search engine friendly.
When ever user search on search engines like Google or Bing, search engine bots starts crawling on each website and collect relevent information based on the keyword searched and arrange this information in index, based of some ranking algorithms the google ranks the searched web page links in a order and show up in the results. higher the rank the page will be shown in the top index.