What is push notification?

Push notification-

Push notifications notify a user of new messages or events even when the user is not actively using your application.

On Android devices, when a device receives a push notification, your application’s icon and a message appear in the status bar. When the user taps the notification, they are sent to your application.

Notifications can be broadcast to all users, such as for a marketing campaign, or sent to just a subset of users, to give personalized information.