The description meta tag is, on its own, a rather unimportant addition to any web page. It does nothing to your pages search engine optimization strategies, in fact it is pretty much ignored by search engines, until your site ranks.
One your site is appearing in the search results, the description meta tag becomes important. When your page comes up in the search results, the snippet of information about your site is taken from your description meta tag. If there is no tag, the search engines find an appropriate piece of text of their own.
Where you can get into difficulties is if you try to over optimize your description meta tag. If you do, Google for example, will ignore it and create its own description. This could look pretty poor if you could see it for yourself. In the long run, a good description meta tag is part of your search marketing. It helps to sell your site to the searchers. They see the snippet, yes thats what I want, and click through to your site.
A poor tag means they will skip past your entry in the search results. Enough searchers do that and you will start to slide down the search results rankings. The moral – treat your description meta tag as a mini piece of advertising copy.