Reducers are functions used in programming, particularly in state management libraries like Redux. They take the current state and an action as inputs and return a new state. This process helps manage how the application's state changes in response to different actions, ensuring that the state remains predictable and consistent. In a Redux application, reducers are pure functions, meaning they do not have side effects and always produce the same output for the same input. This makes it easier to debug and test the application, as developers can track how the state evolves over time based on the actions dispatched.