SharePoint, starting with version 2010, has a feature called Tags and Notes that allowed for ad hoc tagging of content to encourage people to add additional metadata to make it easier to find relevant content. This feature was transitioned into SharePoint 2013 and Office 365.
As of now, the SharePoint Online Tags and Notes feature has been officially retired. If you have existing tags and notes they will be available but disabled. Note Board and Tag Cloud web parts are also now effectively disabled.
Microsoft encourages you to move to Yammer as a replacement for this existing feature. If you want to export all your existing content, Microsoft allows you to export your existing content to a .CSV file.
