Apps send you notifications to let you know about new messages, calendar events, and alarms.
You can check your notifications from a few places:
You see them when you swipe the status bar down on any screen.

If you get multiple notifications from an app, they'll be grouped together. You can swipe down to view more details.
Some important notifications will pop up over your current app so you can take action. For example, if you get a call while you're watching a video, a notification will appear so you can answer or ignore the call.
If an app icon has a notification badge, you can touch & hold it to see the most recent notification:


If you touched your phone to wake it but haven't unlocked it yet, you can do more than just peek at notifications. You can respond, open apps, and dismiss notifications.

To see your notifications when your phone is unlocked, swipe the status bar down on any screen.
Many notifications include actions at the bottom that let you complete common tasks without opening the full app. If you have several notifications, these actions are hidden. When you see 
If you're on the lock screen and Safe reply is on, unlock your phone to continue the action.
From the status bar:
From the awake lock screen:
Tap the notification twice and enter your PIN, pattern, or password to unlock your screen and open the app.
Tap 

To dismiss one notification, swipe it sideways. Some notifications, like voicemails, require an action and cannot be dismissed.
To dismiss all of them, swipe to the end of your notifications and tap Clear all.
After you clear or snooze notifications, you can go to your notification history to view them again.
To set it up:

To see the history, go to Settings > Notifications > Notification history.
Use Do Not Disturb to silence all notifications when you don’t want interruptions.
Touch & hold a notification to customize that app's notifications. Then you can: