Firebase Realtime Database is a cloud-hosted NoSQL database from Google in which data is stored as JSON and synchronised in realtime to every connected client. Changes reach devices within milliseconds, and the SDK keeps data on the device so that apps continue to work offline. Mobile apps and browsers can talk to the database directly without an intermediate application server, with access controlled by Security Rules. It is delivered as a managed cloud service, with SDKs for Apple platforms, Android, web, Flutter and Unity, among others, plus a REST API.
Google is a US company, and Firebase is a proprietary service operated by Google; there is no open-source edition and no self-hosting option. When you create a database you choose its location: us-central1 (Iowa), europe-west1 (Belgium) or asia-southeast1 (Singapore). Choosing the Belgian location keeps the data in the EU, but the provider remains a US company. For European organisations this matters when assessing who holds legal control over the data, even where the physical storage is in Europe.