Search Engine Optimization is the art and science of attracting targeted audiences to your website through Google searches.
Google looks for over 200 different criteria in a website to determine how high it will rank in search results for each of its users based on what key words they use.
SEO work is the constant job of building links to your website on other websites. The larger the site and the higher number of links, then the higher your website will rank.
It is also a lot of work doing market research to find out what keywords are trending and modifying the website to rank better with those key words and be shown on more people's Google searches.