Notification icons report the arrival of new messages, calendar events, alarms, as well as ongoing events, such as when you’re in a call.
When you receive a notification, the notification icon will be displayed in the Status Bar. You can open the Notifications panel to view a list of all of your recent notifications. Depending on your settings, you may hear a notification sound, and/or see a notification light (at the upper-left corner of the phone), and/or the phone may vibrate.