Inertia.js is a framework that bridges traditional backend frameworks (Laravel, Rails, Django) with modern front-end frameworks (React, Vue, Svelte). Instead of building an API and a separate single-page app, you return data and a component name from your backend, and Inertia renders the corresponding front-end component. The result is a monolithic architecture (backend and frontend deployed together) with modern front-end UX. You keep the simplicity of traditional web development while leveraging React/Vue reactive components.