The complexity of search engine optimization can rank anywhere from boiling a pot of water to the theory of relativity. Preferring the former over the latter, maybe a simplification can yield benefits to those who are curious but have not yet embarked upon the SEO journey for their website.
In simple terms, search engines rank websites on their respective relevance to a particular search term used. For example, if you were an insurance agent and typed "insurance agency leads," a listing of relevant sites would be rendered by a search engine, most notably Google, Bing and Yahoo.
How search engines determine this relevancy is the source of much speculation and consternation. Some say Google uses more than 200 variables when ranking results to assess which sites match a specific keyword phrase. Of these 200 variables, several seem to hold more importance than others. These include: domain name, page title, H1 header tags, keyword density and blog and backlinks, the links from other sites to your website.