Search engine optimization (SEO) is a digital marketing technique and it is a process to increase the ranking of websites or a web page in search engine result.
According to Wikipedia
Search engine optimization (SEO) is the process of affecting the visibility of a website or a web page in a web search engine's unpaid results—often referred to as "natural", "organic", or "earned" results.
SEO is most important part of digital marketing strategy because it brings tons of relevant traffic (Organic) on most of the websites. Here organic traffic means unpaid traffic from search engines such as google.com, bing.com, and yahho.com.
Organic or natural traffic is the traffic which any website gets from a search engine. This is basically unpaid traffic which a website gets because of its visibility in search engine ranking on certain keywords.
Inorganic or paid traffic is traffic which a website gets from running a PPC (Pay Per Click) ads of the search engine.