Notes (to be renamed Comments) is a helper app that integrates with other apps (e.g. Users, Requests) and allows users to participate in running commentary on records. You can create comments, reply to comments, tag others in comments etc.
See this post for more details on the vision: https://discuss.folio.org/t/notes-ux-iteration-2/880
While a basic implementation of Notes has already been developed, there is more work to be done to make it functional. There are bugs (e.g. you cannot currently edit or delete notes), and incomplete features. Basic tagging of users is in place, but it offers all users, not just FOLIO operators. And the in-folio Notifications feature where one can view their alerts (namely, have I been tagged in a Note) is extremely rudimentary.
This epic represents the work remaining to complete the Notes feature.