Focus mode is a way of viewing all the comments on your video in a clean environment that removes all other distractions. It's great if you've received a whole lot of feedback and you need to process it systematically. When you're in Focus mode you can still reply, complete, like, edit and delete comments.
You can access focus mode via a button at the top right of the to-do list.
It will also automatically become available via a special button that appears anytime you have a long comment stream that is longer than the available space on the video.