SEO or Search Engine Optimization is a term used to describe practice and methods to optimize a web site for better representation in search results. These practices and methods are usually based on the rules and best practices provided by the major search engines such as Google, Bing, Yahoo! and other major search engines. The guidelines from most of the major search engines are pretty similar to each other. They generally promote use of organic backlinks, relevant and unique content, genuine discussions on the topic and discourage spam links, posts, and contents.
The purpose of these best practices is to make web pages more easily discoverable by search engines and proper indexing of the web pages of the information. 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 discoverable 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.
Some of the things that can affect SEO include: web page title (Article or Post Title in most cases), the URL structure (how descriptive it is), length and importance of content, and many other smaller factors.