Apps send notifications to alert you about things such as new messages, calendar events and alarms.
You see notifications:
When you swipe the status bar down on any screen

If you receive multiple notifications from an app, they're bundled together. Swipe down to view more details.
Some important notifications pop up briefly over your current app so that you can take action. For example, a notification for an incoming call might appear while you're watching a video, so that you can answer or dismiss the call.
Use Do not disturb to silence all notifications when you don't want interruptions.
You can also customise notifications, controlling the notification sound, which apps send notifications and how notifications show on your lock screen.