Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages and notifications at no cost.
Using FCM, you can notify a client app that new email or other data is available to sync. You can send notifications to drive user reengagement and retention. For use cases such as instant messaging, a message can transfer a payload of up to 4KB to a client app.
Firebase Notifications is built on Firebase Cloud Messaging. It is a free service that enables targeted user notifications for mobile app developers.