By default, Focused Inbox is enabled.  Focused inbox helps you focus on emails that matter to you.  It separates your inbox into two tabs, Focused and Other

1.    Click on Settings (gear) in upper right hand corner
2.    Click on Mail link (at the bottom)
3.    Under Layout (on left hand side) click Focused Inbox
4.    Click on Don’t sort Messages
5.    Click Save