SEO is an abbreviation for "Search Engine Optimization" and refers to methods used to increase the number of visitors of a website by improving its rankings in the search results. Any traffic that you do not have to pay for, such as traffic from search engines is called "organic traffic".
Trying to manipulate rankings using SEO techniques which are considered as artificial is called black hat SEO and can be affected by a penalty which may result in the drop of rankings all across the site.
Google dominates search market and rank websites in their search engine results by using an algorithm
called PageRank. According to Google, PageRank works by counting the number and quality of links to a page to determine a rough estimate of how important the website is. The underlying assumption is that more important websites are likely to receive more links from other websites.
Here is a study by ahrefs that describes how "On Page SEO" and backlink factors correlate with rankings:
Backlink Factors vs. On Page Factors:
Today, most people are searching on Google using a mobile device. In November 2016, Google announced a major change to the way crawling websites and started to make their index mobile-first, which means the mobile version of your website becomes the starting point for what Google includes in their index. It is not a mobile-only index, for instance, if a site doesn’t have a mobile-friendly version, the desktop site can still be included in the index.