If video cards,comments, and chapters are enabled for your account, you will see this square icon in the bottom left hand corner of your video.
Cards, comments, and chapters will all appear as annotations in live time for viewers. You will first need to enable them under the video permissions settings:
Cards: Only the owner of the video can add cards. Cards are useful for adding an afterthought, clarification, or an external link.
Comments: The video owner and any viewers linked to the same organization as the owner can leave a comment. To enable commenting for viewers, the owner should check the setting under Permissions. Comments are intended to be interactive. Use them to ask your viewers questions and allow them to respond back with their own comment.
Chapters: Only the owner of the video can add chapters. Chapters are useful for marking different sections of the video, that way viewers can easily jump to the section they would like to watch.