A push notification is a system to send notifications to iPhone and Android devices even if the user is not launching the smartphone application.
Currently, iOS uses the APNS framework, and Android the GCM. Both ask for the user’s permission before sending the notification to the Apple/Google server. The server then forwards the notification to the operating systems, which will be displayed on the screen or within the application.
By using the “push notification” function, if there is some sort of updated information, you can notify the update to multiple users at once.