In the past this tags was used to specify the text that will appear in search engine results alongside the link to your site. Keywords could be placed in this text to influence the ranking. However, it was abused similar to the META Keywords and struck the same fate, which means it is not as useful as it once was. Some search engines will not appear in the results (including Google, which retrieves the text of your page content) and not to use it to rank a site. Having said that some search engines continue to use it again it is a mistake to ignore it completely.
Descriptions placed into this tag should strangely enough be very descriptive of the page or site they reside in. Include a few relevant keywords / key phrases to optimize search engine result and the description and try to limit its duration to 25 -30 words. Also, try not to use more than two sentences. While not as important as in the past but meta tags can still influence a search engine when it comes to deciding whether your web page is relevant or not. Now you just have to find some good key words for your niche and get started.