At any other time during the video, viewers will see just the “i” icon appear when they hover over the player on desktop or whenever the player controls are showing on mobile. They can click or tap this icon to browse all of the cards included in the video.

http://venturebeat.com/2015/03/16/youtube-launches-cards-for-linking-viewers-to-other-videos-playlists-merchandise-fundraising-and-websites/