The Data Monitor measures data traffic consumed by your WiFi and cellular interface. You are able to check when and where you consumed data traffic over your cellular or WiFi radio. Please tap on the entries in the Data Usage view in order to see your data consumption localized on a map.
In order to enable a reasonable time resolution and send you a message in case you are going to exceed your data limits, the Data Monitor needs to stay alive in background. So please launch the Traffic Monitor again after reboot in case your device has been switched off.
The device view allows you to check your device’s status as, e.g. CPU load, memory load, running processes.