SEO stands for Search Engine Optimization. It is the process of getting visibility of a website in the unpaid, so called “organic”, “natural” or “free”, search engine result. The higher a website ranks the better is its visibility. And when people search for something they mostly don't even go to the second page of Google's search results anymore. So of course everyone wants to be ranked on the first page of Google.
Organic search result means, everything that comes up when you type in a search word in Google and does not have the little AD sign in front of it.
SEO can not be done quickly and over night by just optimizing your website and you are done. But instead it is a long term process of consistency. However you can set up your site optimized for SEO.