SEO also known as Search Engine Optimization is to optimize your website that is Search Engine Friendly. It is the process to get significant free, orgnaic traffic such as Major Search Engine like Google , Bing or Yahoo.

Factors that affect SEO

Search engine like rich, high quality content and is consistently improving and updating their algorithm. The few factors that could be easily change to get a better user experience for your viewers
Is your website loading very slow? Considering optimize your images, using a CDN, minify your javascript, css files.
Search Engine love websites that is relevance to the searcher, a site that is rich in content, provide consistent and useful information or service relevant to the searcher will generally be rank higher as they are important site
Tools like Google Analytic is available free, it provide general information about your visitor bounce rate, average page visit size, their demographic and pattern which are useful information to provide your website.
Creating your presence in popular social media site like Facebook, Twitter, Google + page is important as they help to reach your website to more people who are interested in your website product, service or information.
Sites that was created in the early days that did not revamp, are not responsive to Mobile visitor. Smart objects such as Adobe Flash are not able to display on most mobile device