Often a website which hosts videos have a common top-level page that groups conceptually related videos together. Sites with many videos about a single subject can group these videos together on a top-level page, often known as a gallery. This can make it easier for users to find exactly what they’re looking for.
“In this case, you can use a Sitemap to tell Google the URL of the gallery page on which each video appears. You can specify the URL of the gallery level page using the optional tag <video:gallery_loc> on a per-video basis. Note that only one gallery_loc is allowed per video,” explains Google.