Cards are a great way to increase engagement with your video ad or brand.
Where do you create cards?
Cards are created in YouTube and show on a per-video basis, on both desktops and mobile devices. You can provide a final URL from a list of eligible sites and, depending on the card type, customize an image, title, and call-to-action text.
Read more here: https://support.google.com/youtube/answer/150471