I noticed that I just got this feature recently. I think I got it at the same time my channel was enabled for Fan Funding. You can have the card pop up a link to fan funding, a fundraising site (like Patreon, Change.org, kickstarter, indiegogo, and several others), and a merchendise site like Spreadshirt. A full list is available in the google help article titled “Link to fundraising sites through annotations” which I can’t link to.
Personally I think this is really cool and I’ll definitely be using it in my videos. It’s much better looking than a regular annotation. For a live example you can find the video on my channel, but again I can’t link to it.
This pops up when you go to edit settings in a video:
Here’s what the interactivity tab looks like. You can add multiple cards that expand at different times in the video:
What the card looks like, it shows up at the top left of your video:
Hovering over the card expands it. It also expands for a few seconds at the time you pick in the settings:
Here are what the cards look like. Clicking on the icon when there are multiple cards will show them both, but one is highlighted in the middle.