Open Graph
Also known as: og tags, social meta tags
A set of meta tags declaring a page title, description, image and type for systems that render a preview of a link.
Open Graph was built for social previews and has outlived that purpose. It is now a second, redundant statement of what a page is, parsed by a wide range of systems that have nothing to do with social media.
Redundancy is the point. When a parser trips on the primary markup, the duplicate statement is what saves the page from being described wrong.
The og:image is worth more than it looks. Every share of a page without one is a grey box, and a grey box gets a fraction of the clicks a real image does. That is not an SEO effect, it is a human one, and it compounds over every share the page ever gets.