Redux is a powerful state management library for JavaScript applications, commonly used with libraries/frameworks like React. It provides a predictable state container for managing the application's state and helps to make state mutations predictable by imposing certain restrictions on how and when updates can occur. Redux operates on a unidirectional data flow principle, where the state of the application is stored in a single immutable state tree, and changes are made by dispatching actions, which are plain JavaScript objects describing the change. These actions are then handled by pure functions called reducers, which specify how the state should change in response to the actions.