SEO is an acronym for Search Engine Optimization.  It is the art and science of ranking high on search engines such as Google, Bing, Microsoft Edge, Yahoo, etc.  The short is that you want to be on the fist page for any search engine as most consumers will not make it past this page.  The data shows that 91.5% of all Google searches only go to the first page.  Only 4.8% of searches make it to the second page.  Only 1.1% make it to the third  page.  These means that only 1 out of 100 consumers who perform a search will make it to the third page.  Any lower than this and it is almost certain that the searcher will not make it to the page.  This is where SEO comes into play.  You have to optimize your website to ensure that it shows up within the first one or two pages to have a legitimate chance for your website link to even be clicked on.  Thsi is done through many factors such as key words searches,