This is a simple static HTML/CSS only site (no JS, no server, etc) built for one purpose: to let you trick your browser (Chrome, etc) into autofilling your own saved CC details into a <form> that looks like an ecommerce check-out form, so you can see the stored number, expiration, and (if applicable) CVV code.
That's your data, and you chose to securely store it in your browser/payment manager. This tool just makes it easier for you to see your own data.
There is ABSOLUTELY NO TRACKING OR SPYING CODE whatsoever on this page. You could just run it locally in your own browser, but I think browsers are more reluctant to autofill sensitive info when you do so. That's the only reason this is a real site.
But it's hosted here on Github-Pages so that you know it's not on some suspicious private server with potentially nefarious tracking going on, and so that you can see all the "code" (again, HTML/CSS only) that's on it.
Just click into one of the fields below to trigger an autofill/prompt. That's it.