Also known as a description Tag, a meta description tag is a short HTML paragraph that provides search engines with a description of a page’s content for search engine Index purposes. The meta description tag is not displayed on the website itself, and may or may not be displayed in the search engine’s listing for that site. Search engines are now giving less importance to meta description tags in lieu of actual page content.
The description attribute
Unlike the keywords attribute, the description attribute is supported by most major search engines, like Yahoo! and Bing, while Google will fall back on this tag when information about the page itself is requested (e.g. using the related: query). The description attribute provides a concise explanation of a Web page's content. This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own description based on the page content. The description is often, but not always, displayed on search engine results pages, so it can affect click-through rates. While clicks for a result can be a positive sign of effective title and description writing, Google does not recognize this meta element as a ranking factor, so using target keyword phrases in that element will not help a site rank better. W3C doesn't specify the size of this description meta tag, but almost all search engines recommend it to be shorter than 160 characters of plain text.