Meta descriptions are HTML attributes that provide concise explanations of the contents of web pages. Meta descriptions are commonly used on search engine result pages (SERPs) to display preview snippets for a given page. You can find more information here.
Adrelevantis' ad tag reads meta description and uses it to match ads.
As a best practice, you should always populate meta description for your pages. Here is an example of meta description.
<head> <meta name="description" content="This is an example of a meta description. This will often show up in search results."> </head>
Posted by Gary Guo on December 28, 2015