SEO Stands for search engine optimization. Technically, Search engine optimization means techniques to improve the visibility of websites or blogs in search engines. It should be kept in mind that Search engine optimization is not a tweak to improve the ranking of websites. Instead, it is improvements of website with respect to uptime, website navigation, relevant and useful content, availabality and security etc.
Search engine bots go from website to website, crawl pages, and gather information about them. It then puts all those pages in an index. Search engines then calculate which pages to show in search results and on which number based on algorithms. The algorithms of search engines depend on many factors.
Read the following useful articles for further increasing your knowledge about Search engine optimization.