Redux is a predictable state container for JavaScript applications, primarily used with React. It helps manage the state of an application in a centralized way, making it easier to understand and debug. By using a single source of truth, Redux allows developers to track changes in the application state over time. In Redux, the state is updated through actions and reducers. Actions are plain objects that describe what happened, while reducers are functions that specify how the state changes in response to those actions. This unidirectional data flow simplifies the process of managing complex application states.