The META DESCRIPTION tag affects the clickthrough ratio for users that come across your website in search results
The META DESCRIPTION tag is placed in the head section of a web page and it looks like this:
<html>
<head>
...
<meta name="description" content="Your website description goes here">
...
</head>
<body>
...
</body>
</html>
Unlike the TITLE tag, most major search engines ignore the META DESCRIPTION tag when ranking search results. However search engines may use the meta description tag when displaying the result links. The Official Google Webmaster Central Blog says: we do sometimes use the "description" meta tag as the text for our search results snippets
The above means that it is generally a good idea to provide a META DESCRIPTION that contains a meaningful text description. The description does not have to have each and every keyword in it since it won't affect the Google search results, but you should try to style the text in such a way as to increase the chance of a user clicking on your site link after reading the description.