Apple Push Notification service (APNs) is the Apple cloud service that carries remote notifications, also called push notifications, to Apple devices. A push notification is a small message from a company's server to its app on a person's device, delivered even when the app is not running; it can show a message, play a sound or update the app icon's badge.
The app obtains a device token, a unique identifier, and sends it to the company's own server, called the provider server. To notify a user, the provider server sends an HTTP/2 POST request over TLS with a JSON payload and the device token to APNs, authenticated with either a signed token or a certificate. APNs validates the request and delivers the notification; if the device is offline, it may store the notification for up to 30 days. A web Push Notifications Console sends test notifications and shows delivery metrics, and broadcast push updates Live Activities for many users at once.
Apple lists no per-message fee. The key that signs the authentication tokens comes from the company's Apple developer account, and shipping the app requires the paid Apple Developer Program.