The Metal Keycard is a PDF file that you were instructed to download when creating your Metal Vault account. The auto-generated PDF file contains the email you used for signing up for Metal Vault. This is how the filename looks like:
"Metal Keycard for email@example.com"
So in case you need your Metal Keycard at some point to recover your password, you can search for that filename on your computer to find it, if you can't remember in what folder you stored it.
And below is a picture of what the Metal Keycard PDF file looks like when you open it in your PDF viewer of choice. Most browsers have a PDF viewer built-in so you can also click on the right mouse button to open and view the keycard in your browser that way.