With the icons on the status bar at the top of the screen you can get information for example about network connectivity and signal strength, battery power and time. When you want the status bar to display, tap the top of your screen. The status bar will also appear momentarily when things change on your phone. This way you can get specific information about your phone. For example, if your battery is running low, the status bar will display with the low battery icon on it.
If you need more information about the icons on your Lumia, go to the Windows Phone support site, and then use the search field on this page to find the article: What do the icons on my phone mean?