Explain the mechanism of the push notification system.

The push notification system works as follows:

1. The app on the device registers with the push notification service (e.g., APNs for iOS) and receives a unique device token.

2. This token is sent to the application's server.

3. When the server wants to send a notification, it creates a message and sends it along with the device token to the push notification service.

4. The push notification service delivers the message to the device using the specified token.

5. The app on the device receives the notification and displays it to the user.

Thus, push notifications allow the server to asynchronously send messages to users' devices through a centralized service.