You can add or remove a video from the corresponding list by clicking one of the three icons below the videos in each chapter. If a video is added to the list, the icon will change color from gray to blue.
You can save a video to the following three lists:
Favorites
Already Watched
Watch Later
Remove a video from list:
If a video is already in a list (blue icon), you can remove it by clicking on the icon. After deleting it, the icon color will change from blue to gray.