Firebase Cloud Messaging is a cross-platform messaging solution that lets you reliably deliver messages and notifications to Android, iOS or the Web at no cost. For example, you can specify that new data is available for sync, special offers are ready to re-engage users and more. Messages can be sent to individual devices, groups of devices, or even topics that devices are subscribed to.
Messages can carry a payload of up to 4k, and can also be sent upstream from devices to a central server or other devices.
Firebase Cloud Messaging is the successor to Google Cloud Messaging, and you can learn details here about your options if you already use Google Cloud Messaging.