The Meta Description Tag (placed between the head and /head tags) is used by some search engines to create the snippets of text that you see right below the results on search queries. It does not carry any weight in the search algorithm of Google, but it does carry a certain weight for Yahoo and other smaller search engines so you should not neglect it altogether.
One problem with the Meta Description Tag of most websites is that it is static. People usually include a general description about the site, which is suitable only to the home page. Should a visitor find an internal page (say a single post) through a search engine the tag will be the same, containing no information about the content of that internal page whatsoever. You can use the SEO tools listed here to check the quality of your meta description tags.
In order to optimize your Meta Description Tag you should make it dynamic, making sure that it will include the first few lines of text of every single page on your site. A very simple way to do this under the WordPress platform is to make the Description tag equal to the post excerpt, like the code below illustrates.