What Open Graph tags do
When someone shares a link on Facebook, LinkedIn, X, Slack, WhatsApp or iMessage, the app reads the page's Open Graph tags (og:title, og:description, og:image) to build the preview card. X also reads its own twitter: tags and falls back to Open Graph.
Without these tags, apps guess: they may pick a random image, cut the title, or show no preview at all. A clear title and a good image make far more people click a shared link.
What this checker looks at
It reads all og: and twitter: tags, downloads the share image to check that it loads and to measure its real size and shape, compares og:url with the canonical tag, and draws the preview the way each app shows it.
The best share image is 1200×630 pixels (about 1.91:1), a JPG or PNG under 5 MB, at a full https:// address.
When previews don't update
Social apps keep a copy of each preview. After you change your tags, refresh it with Facebook's Sharing Debugger and LinkedIn's Post Inspector; X updates on its own after a while.