The tags a preview is built from
Open Graph is a small vocabulary of meta tags. The ones that change a link card most often are og:title, og:description, og:image, and og:url. og:type describes the kind of object, such as website or article. og:site_name names the site as a whole, which some products show as a secondary label.
These tags sit in the HTML <head>. They are suggestions. A platform can truncate a long title, crop an image, or keep serving a card it fetched last week.
What this page checks
The analyzer above requests the URL on the server and lists every Open Graph value it can find, alongside the document title, meta description, and canonical URL. Missing values stay visible as missing. The tool does not invent a title to make the card look finished, though the estimated cards will say when they fell back to the HTML title or meta description.
What a preview cannot see
If the tags are created in the browser after JavaScript runs, a crawler that only reads the first HTML response will miss them. The same limit applies here. Pages behind a login, and hosts that refuse automated requests, return an error instead of a partial guess.
When the image itself is the problem, use the OG image checker. For platform-specific notes, see the Facebook, LinkedIn, and X card guides.