The blue bar isn't the only change, though. On previous iOS versions, users had the option to select whether a given app was "Always" or "Never" allowed to use your location for the app's functionality. It made no distinction -- unless the app's developer had chosen to make the distinction -- of whether or not location tracking was happening in the background. On iOS 11, any app that uses your phone's location services gets a third option, "While Using App," an update which will make it impossible for apps to track your location in the background if you don't want them to. Only some apps included this option previously.
Additionally, the previous signal that an app was using your location data -- the tiny solid and hollow arrows at the top of your iPhone's display -- is changing too. On iOS 10, the hollow arrow lit up and turned solid, and stayed solid, if any app was using your location data, whether it was just a quick ping or movement outside of a given place (i.e. apps using geofencing for music festivals) or continuous location tracking (i.e. Google Maps). That was flawed, Apple reasoned, because it mischaracterized what location data was being sent from certain apps as opposed to others. On iOS 11, the arrow remains, but it will appear and display as hollow when a location is requested. When the location is given, it will then turn solid for a few seconds.