SEO or Search Engine Optimization is a process to get organic or free traffic from search engine results that will affect the visibility of a website on search engine.
To increase the visibility of your website there are two main traffic you need to get:
It means you need to have a relevant content on your site to get quality traffic. Your relevant content should attract visitors to enter to your site. When a visitor come to your site because they interest with what you offer you can call it a quality traffic.
When visitor type certain keyword like "web design" for example, the search results will appear with bunch of links pointing to any website related to "web design" as a search keyword. When they click one of them and get any information according to their needs, you get quality traffic.
The amount of visitor visiting to your site. When you get the right people (quality traffic) from search engine result, you need more visitor to click to your site. The more you get the better position in Search engine results page you will get.
The benefit when you get top position in organic search results is you don't need to pay to get into top position.
If your website appear on first page of Google, in one of the top 3 organic position you will get at least 58.4% of all clicks from users.
If your website ranked number one the average click-through-rate (CTR) is 36.4%, number two get 12.5% and number three had 9.5%
Here is the list of the average CTR in Google Search Engine: