Indicator icons or Status symbols. The bar at the top of the screen tells you the current time, battery and signal strength, and much more. The following screenshot shows the icons for Nokia Lumia 820:
The type of mobile network you’re currently connected to may be shown with a single letter, a combination of letters, or a combination of letters and numbers. For example, 
, depending on your network service provider.
The icons may vary depending on your region or network service provider.