Firebase handles the backend online element for your apps, allowing you to focus on the front-end UI and functionality. All this is done through a single SDK with easy-to-use APIs and excellent integration into Android Studio. This removes the need to create your own server-side script using PHP and MySQL, or a similar set-up. This is ‘Backend as a Service’ or ‘BaaS’, and essentially this means that anyone really can make that ambitious social app. It works with Android apps, iOS apps and web apps and best of all: it’s free!
Firebase has Different features:

Realtime Database

• The Firebase Realtime Database is a cloud-hosted NoSQL database that lets you store and sync between your users in realtime.
• The Realtime Database is really just one big JSON object that the developers can manage in realtime.
• The Realtime Database can also integrate with Firebase Authentication to provide a simple and intuitive authentication process.

• Firebase Cloud Messaging (FCM) provides a reliable and battery-efficient connection between your server and devices that allows you to deliver and receive messages and notifications on iOS, Android, and the web at no cost.
• You can send notification messages (2KB limit) and data messages (4KB limit).
• Using FCM, you can easily target messages using predefined segments or create your own, using demographics and behavior. You can send messages to a group of devices that are subscribed to specific topics, or you can get as granular as a single device.
• FCM can deliver messages instantly, or at a future time in the user’s local time zone. You can send custom app data like setting priorities, sounds, and expiration dates, and also track custom conversion events.
• The best thing about FCM is that there is hardly any coding involved! FCM is completely integrated with Firebase Analytics, giving you detailed engagement and conversion tracking.