A Meta Description is specific type of meta tag which describes a page in a short passage of text. The Description is most commonly seen in the results of a search engine.
For example, in this result from Google:
The black text seen here comes from the Meta Description tag for this page.
Specifying a Meta Description tag
A competent Content Management System should do this for you, but if you need to specify a Meta Description in HTML, the code looks like this:
<meta name="description" content="Silktide specialise in website testing and consulting. We've been helping organisations improve their websites since 2001." />
Note that the end slash shown here is for XHTML only.
Sitebeam tests for the appropriate use of Meta Descriptions with the Search engine results test.