Firebase is a toolset to “build, improve, and grow your app”, and the tools it gives you cover a large portion of the services that developers would normally have to build themselves, but don’t really want to build, because they’d rather be focusing on the app experience itself. This includes things like analytics, authentication, databases, configuration, file storage, push messaging, and the list goes on. The services are hosted in the cloud, and scale with little to no effort on the part of the developer.

Build your app — creating the “guts”
The “build” group of products are these:

Authentication — user login and identity
Realtime Database — realtime, cloud hosted, NoSQL database
Cloud Firestore — realtime, cloud hosted, NoSQL database
Cloud Storage — massively scalable file storage
Cloud Functions — “serverless”, event driven backend
Firebase Hosting — global web hosting
ML Kit —SDK for common ML tasks

Firebase Authentication takes care of getting your users logged in and identified. This product is essential to getting some of the other products configured properly, especially if you need to restrict access to per-user data (which nearly every app will want to do).