This is the metatag that site's such as Facebook use for link preview images.
(See the sample image to the right for an example, image credit goes to )
Without this hook enabled, whenever someone uses a site like Facebook to link to a file hosted on your website, this is the preview image they will see:

This is the default for IP.Board. If you allow screenshots with your file submissions, you probably don't want that. That's where this hook comes in. It simply parses your files featured image and uses it instead.
Once installed, you can use Facebook's debugging tool to ensure it's working. Just paste a link to any of your files. Assuming you have a preview image included with that file, you should see it first under og:image. (You will still see the IP.Board logo after it)