Here is a basic explanation:
Search engine optimization (SEO) is the process of affecting the visibility of a website or a web page in a search engine’s unpaid results-often referred to as “natural,” “organic,” or “earned” results. In general, the earlier (or higher ranked on the search results page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine’s users, and these visitors can be converted into customers. SEO may target different kinds of search, including image search, local search, video search, academic search, news search and industry-specific vertical searchengines.
As an Internet marketing strategy, SEO considers how search engines work, what people search for, the actual search terms or keywords typed into search engines and which search engines are preferred by their targeted audience. Optimizing a website may involve editing its content, HTML and associated coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines. Promoting a site to increase the number of backlinks, or inbound links, is another SEO tactic. As of May 2015, mobile search has finally surpassed desktop search, Google is developing and pushing mobile search as the future in all of its products and many brands are beginning to take a different approach on their internet strategies SOURCE: https://en.wikipedia.org/