Redux is a state management library for JavaScript applications, commonly used with React for building complex user interfaces. It provides a centralized store to manage the state of an application, ensuring that the state is predictable and can be easily tracked and debugged. In Redux, state changes are made using actions and reducers: actions are plain objects describing what happened, and reducers are functions that determine how the state changes in response to these actions. This predictable state container allows for a more structured and maintainable codebase, particularly in large-scale applications. Redux's principles of a single source of truth, state immutability, and unidirectional data flow help developers manage state consistency and enhance the reliability of their applications.