Basically Google uses a complex mathematical formula called an algorithm to give a score to every website and every search people to do in Google to figure out which website should rank best for what people are looking for. Think of the algorithm like a collection of empty buckets. One bucket gives you a score for the quality of your site, one bucket gives you a score for how many sites link to you, one bucket gives you a score for how people trust you. Your job is to fill up more buckets in the algorithm than any other website. You can affect your search engine ranking by having the highest score in terms of quality of your site, of having the highest score in terms of authority of your website, of having the highest score in terms of the most trusted store for that search that people are looking for. The good thing is that there are hundreds of buckets, and for every single one of these buckets these scores put together in the algorithm to figure out where you rank is an opportunity for you to fill it up and rank better. So optimizing your site for search results really means getting the highest score in as many of these points as you can.
Search engine optimization is a key part of online marketing because search is one of the primary ways that users navigate the web. In 2014, over 2.5 trillion searches were conducted worldwide across search engines such as Google, Bing, Yahoo, Baidu, and Yandex. For most websites, traffic that comes from search engines (known as "natural" or "organic" traffic) accounts for a large portion of their total traffic.