Firebase Cloud Messaging (FCM) is a cross-platform messaging service in Firebase, Google's platform for building mobile and web apps. It delivers push notifications, messages that appear on a phone or in a browser even when the app is not open, and data messages that the app's own code handles.
An FCM setup has two parts. A trusted environment, such as Cloud Functions for Firebase or the business's own app server, builds and sends each message through the Firebase Admin SDK or the FCM v1 API. A client app on iOS, Android or the web receives it through the platform's own transport service. A message can go to one device, to a group of devices, or to all devices subscribed to a topic, and can carry a payload of up to 4096 bytes. A Notifications composer in the Firebase console sends messages without code, schedules them in each user's local time zone, and reports engagement through Google Analytics.
It is a hosted service, and the page states that there is no cost to using it.