SEO stands for “search engine optimization.” It’s the process of getting traffic from the “free,” “organic,” or “natural” search results generated by search engines.
Google and Bing are the biggest search engines, and they use algorithms to examine the content on a page to decide what the page is about. Then based upon more than 200 factors, they decide how relevant that page is to certain keywords.
The job of a search engine like Google is to find content that matches your query — the basic question you are asking, like:
"How far is the earth from the sun?" "Who is the leader singer of Led Zeppelin?" "What is a freemason?"
Those questions contain keywords. And so the better your content can match those questions, the better the experience for the user. When you make people happy, you make Google happy.