What a link preview is
When you paste a link in WhatsApp, Facebook, LinkedIn, X, Slack or iMessage, the app visits the page and builds a small card: an image, a title, a line of description and the site's name. That card is often the only thing people see before deciding to tap.
The apps build it from tags in the page's code — mostly the Open Graph tags (og:title, og:description, og:image) that Facebook introduced and everyone adopted, plus X's own twitter:card. When they're missing, each app guesses, and the guesses rarely look good.
The tags that matter
og:title is the headline: short, written for people, under about 60 characters. og:description is one sentence under the title. og:image is the picture — 1200×630 pixels, a JPG or PNG, under a few hundred KB — and it's the part that decides whether anyone notices the link at all.
twitter:card set to summary_large_image gives you the big card on X instead of a small thumbnail. og:url and og:site_name are small touches: the page's main address and your brand's name.
Why the preview doesn't update
Apps keep a copy of each preview so they don't visit the page every time someone shares it. After you change your tags, the old card can stay for days. Facebook's Sharing Debugger and LinkedIn's Post Inspector let you ask for a fresh one; on WhatsApp and X the simplest trick is a slightly different address, such as adding ?v=2.
Common mistakes
A logo as the preview image. A small square logo becomes a tiny thumbnail or gets cropped. Use a wide 1200×630 image with your logo inside it.
The same preview on every page. When every product shares the home page's image and title, people can't tell the links apart. Give each important page its own.
Images behind a login or blocked for bots. The apps' visitors aren't logged in; if the image isn't public, the card has no picture.
How the check works
Our server visits the address once, the way the apps do, reads the tags in the page's <head> and downloads the preview image to measure it. Nothing is stored, and the image is shown to you as a copy, so your browser doesn't load it from the site.