Believe it or not but by default, WordPress doesn't add any Meta descriptions to the posts you write. Sure, in terms of SEO, Meta descriptions aren't as important as they used to be, but good Meta descriptions are always a plus for both your visitors and search engines bots.
The HTML Meta tag is used to tell things about the web page to search engines crawlers. The description attribute is used to give a short description of the web page. Actually, it is supported by all major search engines such as Google, Yahoo, and MSN/Live search.
The bigger advantage of this tag is that it gives you control over the description that will be displayed on search engines' results pages, because the Meta description is often—but not always—used by search engines to give the visitor a brief description of the page. Without the Meta description tag, the search engine will usually take the first phrases from the page and use it as a description. You guessed...