That's one of the exercise modes, possibly Free Run since I can't see the GPS indication. Whichever one it is, that display type is only used while you're tracking the appropriate exercise. When you finish, it returns to whatever clock face you've selected.
Mike | London, UK
Blaze, Surge, Charge 2, Charge, Flex 2 - iPad Air 2, Nokia Lumia 925 (Deceased), iPhone 6
Take a look at the Fitbit help site for further assistance and information.