SEO which simply means Search engine optimization is the process of improving the quality and quantity of website traffic to a website or a web page from search engines. SEO targets unpaid traffic rather than direct traffic or paid traffic.
Also,they generally promote use of organic backlinks, relevant and unique content, genuine discussions on the topic and discourage spam links, posts, and contents.
WordPress is already a very search engine friendly content management system, however, users can further optimize their WordPress powered websites by modifying certain settings or by using WordPress SEO plugins.
Improving the SEO of a website would only make it more easily discover able and properly indexed by search engines. It is not necessarily true that a website with good SEO practices would gain better results in search engines or get more traffic. However, search engines are still a vital source of traffic for many websites so it is important that their websites follow these SEO guidelines.
For example, Google has over 200 ranking factors in their algorithm.