Firebase is Google's unified platform for building mobile app backends. Instead of writing servers, you use managed services: Firestore (real-time database), Cloud Functions (serverless compute), Cloud Storage (file storage), and Hosting (deployment). Declare data structure, add security rules to control access, and your app works. Firebase handles scaling, redundancy, and security. No backend engineering required. Mobile developers call Firebase APIs directly from their apps. Data syncs in real-time: user edits note on phone, iPad sees change instantly (zero-latency sync via WebSockets).