Simply put, search engine optimization (SEO) is the process of optimizing the content, technical set-up, and reach of your website so that your pages appear at the top of a search engine result for a specific set of keyword terms. Ultimately, the goal is to attract visitors to your website when they search for products, services, or information related to your business.
Not every single ad will appear on every single search. This is because the ad auction takes a variety of factors into account when determining the placement of ads on the SERP, and because not every keyword has sufficient commercial intent to justify displaying ads next to results. However, the two main factors that Google evaluates as part of the ad auction process are your maximum bid and the Quality Score of your ads.