SEO stands for “search engine optimisation” 
It is the process of making  your website or web page more visible online in the organic search engine results, so increasing the traffic (the number and quality of visitors) and improving rankings in the algorithmic search engine results. These algorithmic search results are ‘natural’, ‘organic’, ‘free’ or ‘earned’ results. 
All major search engines such as Google, Bing and Yahoo have primary search results, where web pages are shown and ranked based on what the search engine considers most relevant to users. Payment isn’t involved.
Websites on the first page of Google receive almost 95% of clicks, and those that appear higher up the page have a higher click through rate (CTR), and more traffic. The higher ranked, on the search results page, and more frequently a website appears in the search results list, the more visitors it will receive from the search engine's users and the more potential conversions from visitor to customer.
There are many ways you can improve your SEO, such as having great content on your page, good use of meta tags and long tail keywords, linking other sites to your website and making sure your site is structured in a way that search engines understand.
But SEO isn't just about building search engine-friendly websites – it's about making your site better for people to use too.