Dictionary
Open Graph
A set of meta tags that control how a link looks when shared on social media or messaging apps.
In more detail
What It Means
Open Graph is a set of meta tags, originally created by Facebook, that control the title, description, and image a link shows when it's pasted into a social media post, a Slack message, or a text conversation. Without them, a shared link falls back to whatever the page's own title tag happens to say.
See it in context on the homepage: The Head.