Before Youtube Cards were implemented by Youtube, when you created a video, you had to put a link as a visual link in the end of your video.
Now, Youtube has realized that there was a need by their customers to have a Call to Action in their videos so they created the Youtube Card.

In the Youtube Card, you can have a link to your website or thumbnails of your other videos that a viewer can click on to watch more videos.
I am creating some trainings on how to setup and marketing using various social media platforms from Facebook to Twitter to Youtube so I will have more information on these trainings later. Have a look at this Google post on what a Youtube Card is!