InstantDB is a modern backend for AI-coded apps, offering a relational database that allows for real-time updates, offline mode, and multiplayer capabilities. It aims to simplify the development process by providing a client-side database that handles data fetching, permission checking, and caching. The platform is designed to be easy to use, with a simple query language and support for JavaScript, React, and React Native. InstantDB is positioned as an alternative to Firebase, with a focus on relational queries and open-source architecture.