Your description tag summarizes your page's content. Some search engines use this description to help categorize your page, and a few even display it in search results!
With this in mind, you should make sure your page's description is both readable and interesting, and also features one or two keywords that appear in the corresponding page.
Be sure your description doesn't exceed 256 characters. (Our Site Solution and SiteBuilder meta tag tools will limit the length of your keywords to 256 characters automatically.) Be sure to include one or two keywords and, if possible, place these near the beginning of your description.
Tip: Having trouble thinking of a description? Try using the first sentence or two from your page.