SEO is short for Search Engine Optimisation. With SEO, steps are taken to improve the quality of your website and its online presence so that it ranks higher in Google.
For Google to decide what web pages to show on a particular search result and in what order they have a very complex algorithm. There are 200+ factors that make up this algorithm such as website speed, quality of information on the website, and who is linking to the website.
These ranking factors are always changing and evolving. You need to be overall better than your competition in these 200+ ranking factors to rank higher than them in the search results.