Very similar to meta tags, eg title / description etc, its just a way to make social platforms read the preferred data rather than guess it.
Here would an example
http://os4depot.net/?function=showfil ... y/benchmark/boingcube.lha<meta property="og:url" content="
http://os4depot.net/?function=showfil ... hmark/boingcube.lha" />
<meta property="og:type" content="article" />
<meta property="og:title" content="BoingCube" />
<meta property="og:description" content="A bouncing rotating boing cube" />
<meta property="og:image" content="
http://os4depot.net/filedata/snapshot/9645_1_snap.jpg" />
<meta property="og:image" content="
http://os4depot.net/filedata/snapshot/9645_2_snap.jpg" />
The og:url could be a canonical url if theres one avaiable
Depending on how the website is built, these would be simple database fields of the uploaded file, some like the image can be mocked up just by the upload file ID even it the image doesn't exist, so by default it would pull the first image on the page.
The description tag could hold more information like filesize, platform, date added, author etc
Twitter has there own set of similar tags
https://dev.twitter.com/cards/markup