What Open Graph is
The Open Graph protocol was introduced by Facebook in 2010 to turn any web page into a rich object in a social feed. It uses meta tags with a property attribute, such as <meta property="og:title" content="...">, placed in the page head.
Today almost every app that unfurls links reads it, including Facebook, LinkedIn, Slack, Discord, WhatsApp, iMessage and Microsoft Teams. X (Twitter) uses its own twitter: tags but falls back to Open Graph.
The essential tags
og:title is the headline of the share card and og:description the line beneath it. og:image is the preview image; use an absolute URL and an image of 1200 x 630 pixels, which suits most platforms. og:url is the canonical URL of the page, og:type is usually website or article, and og:site_name is your brand.
Open Graph does not affect Google rankings directly, but an attractive share card earns more clicks and links. Keep it consistent with your title tag and meta description.
Why the wrong preview shows
Platforms cache previews. After changing tags, refresh the cache with Facebook's Sharing Debugger or LinkedIn's Post Inspector. Other causes are a relative image URL, an image that is too small or blocked by robots.txt, tags added by JavaScript (most scrapers do not run it) and several conflicting og:image tags.
Example
Open Graph and Twitter tags:
<meta property="og:type" content="article">
<meta property="og:title" content="What is a canonical tag?">
<meta property="og:description" content="How canonical tags consolidate duplicate URLs.">
<meta property="og:url" content="https://example.com/learn/canonical-tag">
<meta property="og:image" content="https://example.com/og/canonical.png">
<meta name="twitter:card" content="summary_large_image">