As a search engine prowls your site, it gathers information from the title, headings, content, and Meta Tags such as description or keywords. It compares the words within each of these sections and “ranks” the site dependent upon how well the information matches. We have more information on how to maximize your meta tag references below.
It is important for website developers to understand that a default installation of WordPress does not contain the description and keywords meta tag data. Meta tags can be added manually, through changes to the Theme template files or through WordPress Plugins.
Find more here: http://codex.wordpress.org/Meta_Tags_in_WordPress