Gravatar Hovercards adds a pop-up window over your commenters’ Gravatar images that contains more information about that person.
Many WordPress users and commenters have Gravatar accounts, which allows you to show a personal image and additional information whenever you post a comment on a WordPress-powered website.
With the Gravatar Hovercards feature enabled in Jetpack, you’ll see a card pop up when you hover over their avatar image, containing their bio, contact, information, and other details.
To enable this feature, go to: Jetpack Settings > Engagement and scroll down to Gravatar Hovercards and check that it is enabled. Then, if someone has a Gravatar account, you’ll see their Gravatar Hovercard appear over their image in a comment on your site.
By the way, if you haven’t already created your own Gravatar, you can easily do that today at: Gravatar.com