YouTube Annotations, those boxes that appear on the video with links to other content or websites, will be shelved soon. In its place, YouTube will be introducing two new systems into the mix: Cards and End Screens.
Beginning on the 2nd of May, YouTube will remove the Annotations Editor from the service, meaning you'll no longer be able to add annotations to any of your videos from that point forward. While you can no longer add new annotations to videos, existing annotations will still be visible to those that view YouTube content on the desktop (annotations do not appear on the mobile YouTube app anyway).
For those who rely on annotations to link multiple pieces of content together, the direct replacement for the feature would be Cards. Cards are tiles that appear on the right hand side of the screen when the video is playing. Upon clicking a tile, the viewer would then be brought to the content that is linked to the tile. As for End Screens, this feature would let you add thumbnail overlays of your work in the last 5-20 seconds of your video.
Both Cards and End Screens are options you can use and they can be found in YouTube's Video Manager. Additionally, both features will also appear on the mobile version of YouTube.
Source: YouTube Blog