A callback function is a programming concept where a function is passed as an argument to another function. This allows the receiving function to execute the callback at a later time, often after completing a specific task. Callbacks are commonly used in asynchronous programming, enabling code to run without blocking the main thread. In languages like JavaScript, callbacks are essential for handling events, such as user interactions or data retrieval. For example, when a user clicks a button, a callback function can be triggered to perform an action, like displaying a message. This approach enhances code organization and responsiveness.