The "focused inbox" is enhanced functionality within Outlook, replacing clutter. Emails you need to act on right away are on the Focused tab, while the rest wait for you in Other. You’ll be informed about email flowing to Other, and you can switch between tabs at any time to take a quick look.
To enable/disable the focused inbox for your account, see https://support.office.com/en-us/article/Turn-off-Focused-Inbox-f714d94d-9e63-4217-9ccb-6cb2986aa1b2.