SEO stands for Search engine Optimization. It is the process of enhancing the visibility of a website on organic( i.e. natural way ) search engine result pages (SERPs), by incorporating search engine friendly elements into a website. It can be done in two ways:
On Page Optimizations: It refers to website elements which comprise a web page, such as HTML code, textual content, and images.
Off-page optimizations: It refers, predominantly, to back links (links pointing to the site which is being optimized, from other relevant websites).