Alerts let you know about important events. They can appear briefly at the top of the screen, or remain in the center of the screen until you acknowledge them.
Some apps may include a badge on their Home screen icon, to let you know how many new items await—for example, the number of new email messages. If there’s a problem—such as a message that couldn’t be sent—an exclamation mark 
appears on the badge. On a folder, a numbered badge indicates the total number of notifications for all the apps inside.
Alerts can also appear on the Lock screen.
Respond to an alert without leaving your current app. Pull down on the alert when it appears at the top of your screen.
Note: This feature works with text and email messages, calendar invitations, and more.
Respond to an alert when iPhone is locked. Swipe the alert from right to left.
Silence your alerts. Go to Settings > Do Not Disturb. You can also use Siri to turn Do Not Disturb on or off. Say “Turn on Do Not Disturb” or “Turn off Do Not Disturb.”
Set sounds and vibrations. Go to Settings > Sounds.