MobX is a state management library for JavaScript applications, particularly popular in React projects. It simplifies the process of managing application state by using observable data structures, allowing developers to automatically update the user interface when the underlying data changes. With MobX, developers can create reactive applications where components automatically respond to state changes. This leads to more efficient and maintainable code, as it reduces the need for manual updates and complex state management logic. Overall, MobX enhances the development experience by promoting a clear separation between state and UI.