Notification icons report the arrival of new messages, calendar events, alarms, as well as ongoing events, such as when you’re on 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 the phone may vibrate.
You may also see notifications in the Second screen area if it's enabled in the Settings app.