The Focused Inbox is coming to Outlook desktop users (both Windows and Mac). When it's available in your account, Outlook will pop up a dialog alerting you to the new feature and asking if you want to use it.
If you are unfamiliar with the Focused Inbox, Outlook sorts your mail by what it thinks is important, classifying some mail (mostly bulk mail) as the less important, "Other". This is a special view, not a folder, and the mail remains in the Inbox.
The Focused Inbox in Outlook 2016 for Windows. (Click on All to select Unread)
The Focused Inbox in Outlook 2016 for Mac. In this mailbox, there are 3 new messages in the Other tab.
If you say Yes then change your mind, you can turn the Focused Inbox off from the View tab in Outlook for Windows or on the Organize tab in Outlook for Mac. It's also on the All menu (formerly By [field]) in Windows and on the By: [field] menu in Mac.
If messages are misclassified as either Other or Focused, right click on the message and choose Move to the [Focused or Other].
When Focused Inbox is turned off, the only tab you'll see is Inbox; Unread and Mentioned were removed as tabs but are on the All menu (and work with Focused Inbox enabled).