This means in a video that you’ve shared, maybe for an advertisement or a presentation, other people collaborating on the project can comment at specific times in the video to leave feedback and suggestions. This will also work with audio files shared on Dropbox. Users who leave comments can also tag other users with the “@” tag.
This is similar to other platforms such as Soundcloud where users can comment on specific parts of a song. To a certain extent, other platforms such as YouTube and Facebook have similar features, except that they only work for live videos where you can see which comment was made at which point in the video. It is unclear why this isn’t becoming a mainstream feature as it does seem pretty useful.
That being said, the latest comments feature for Dropbox is available for the web version and iOS version of the service and should be arriving soon. Also while any Dropbox user can comment, only paid Dropbox users will have the feature enabled.