SEO or Search Engine Optimization is a methodology of strategies, techniques and tactics used to increase a number of visitors to a website by obtaining a high-ranking placement in the search results page of a search engine ie. Google, Bing, etc.
It is common practice for Internet users to not click through pages and pages of search results, so where a site ranks in a search is essential for directing more traffic toward the site.
The higher a website naturally ranks in organic results of a search, the greater the chance that that site will be visited by a user.
If you’d like to understand better how this works the following 3-minute video will explain things easily.