Tags that usually matter
The headline comes from og:title, the summary from og:description, and the picture from og:image. og:url should be the canonical address of that page. If the title or description tag is absent, the estimated card on this site falls back to the HTML title or the meta description and labels that fallback.
The image area in the estimate uses a wide 1.91:1 frame, which is close to the crop many Facebook previews use. Put the important part of the picture near the middle. A 1200×630 file gives that crop enough pixels to stay sharp.
Why the live Facebook card can differ
Facebook caches link metadata. Changing a tag on your site does not always change a card that was fetched earlier. Facebook’s Sharing Debugger is the official way to ask for a new fetch. This tool does not talk to Facebook, and the card above is labeled an estimated preview for that reason.
Very long titles are truncated in the feed. Write the og:title as a sentence a person can scan, and keep the description to a couple of lines. If the image URL is relative or returns an error, start with the OG image checker.