Checks for a description meta tag and makes sure it is the proper length.
Every webpage should have a description meta tag in the <head> section of the html. This description is often (but not always) displayed in search results for your website. It should compliment the page title and provide a more detailed description of what is located on the webpage. Try to imagine what your customers would want to know before clicking on your link and write a description for that instead of stuffing the description with keywords for robots to read.
<meta name="description" content="Find the best widget for your gizmo in our online store. Free shipping and easy returns.">
This test will check for the existence of this meta tag and then test it for length. A description that is too short may not accurately describe the contents of the website. A description that is longer than 160 characters may get truncated in search results.
For more information on selecting a good page title, check out the Description Tag page at Moz.com.