Adwords dynamic structured snippets are lines of text generated through Adwords and inserted into ad copy. The dynamically generated ads use content from your landing pages to create the snippets.
According to Google, the snippets attempt to achieve the following:
Whether it’s a list of shoe brands or the number of nonstop flights to New York City, this automated extension gives your customers a better sense of what to expect on your website before they click on your ad.
Unfortunately, many PPC agencies have had trouble inserting the right kind of content into their ads. Because the snippets are dynamically generated, advertisers are unable to choose which elements from their website are going to be harvested for the ads. Many agencies have found that the dynamically generated copy will use navigational elements of their site, like a menu or sub-menu. This results in snippets containing a list of categories, something that many advertisers don’t find helpful for their ad copy. Many advertisers might find it beneficial to wait for improvements from Google before using dynamic structured snippets.