How does S.E.O. work?
Whenever you, as an ordinary person searching for something online, type into Google, or Yahoo, Bing, etc. a certain phrase and then click that 'Search' button, you are in a sense sending off a million 'web-crawlers' that are doing their absolute best to give you the most relevant information for your search. So they go through all of the websites and rank them for you, showing you what they think you're looking for.
Search Engine Optimization is, in the most basic sense, making one's website the most relevant. We do this with several practices, including keyword research, meta tags, schema code, and by utilizing Google Search Console and Google Analytics.