iPhone 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 when iPhone is locked. Swipe the alert from left to right.
Silence your alerts. Go to Settings > Do Not Disturb.
Set sounds and vibrations. Go to Settings > Sounds.