What does this report contain?
All pages with a vary: user-agent value in the headers which indicates to search engines that different HTML might be returned for a different user agent on the same URL.
How can this report be used?
If Googlebot sees the user-agent vary value, it is likely to crawl the same URL using both Googlebot and Googlebot mobile user agents. This may be the correct configuration if a genuine mobile specific page is returned on the same URL as the desktop version of the page.