End Screens help to retain your viewer’s engagement by giving them the option to click on a link, video, playlist, or channel that you want them to be directed to.
End Screens are displayed in the last 5-20 seconds of a video and allow you to add up to four elements (at least one element must be a video or playlist) to promote your content, channel and websites. Each element can expand to show more information when your viewer hovers over it on a desktop computer or taps on it using a mobile device.
If you plan to use an end screen on your video, you will need to add 5-20 seconds of black (or whatever color or image you want as the background for your end screen) to the end of your video before uploading it to YouTube. YouTube will then overlay the End Screen elements that you choose over that last 5-20 seconds of your video.