Can you explain what topics are in a push notification system?

In a push notification system, topics are a kind of channels or groups that devices or users subscribe to in order to receive certain messages. Instead of sending a notification to each device individually, the server sends a message to the topic, and all subscribers to that topic receive the notification.

For example, in Firebase Cloud Messaging (FCM), you can subscribe an app to the "news_sport" topic to receive all news related to sports. This is convenient for audience segmentation and mass mailing.

Example of subscribing to a topic in Android using FCM:

FirebaseMessaging.getInstance().subscribeToTopic("news_sport")
    .addOnCompleteListener(task -> {
        if (task.isSuccessful()) {
            Log.d("FCM", "Subscribed to news_sport topic");
        }
    });