Can you explain what is meant by push notification and how it functions?

Push notifications are messages that the server sends to the user's device without a request from the user. In iOS, they operate through the Apple Push Notification Service (APNs). When the server wants to notify the user, it sends a message to APNs, which then delivers it to the device. The application receives the notification and can display it to the user, update data, or perform another action.

Example: a bank app sends a push notification about a deposit. The user sees the message on the screen, even if the app is closed.