Focused Inbox is an essential feature of the Microsoft Office365 account. It's intelligently to sort your email so you can focus on what matters. It places your most important emails in "Focused" and the rest in "Other." Currently, only Office365 accounts are supported. You can enable the Focused Inbox in the following settings.
OfficeMail - Setting > Select an Office365 account > Email > Focused Inbox > Turn On.
When entering the Inbox of your O365 account, Focused, Other, and All are displayed at the top of the screen, and you can also set the "Focused only" in the popup options appearing when you tap the options menu icon on the top right.