Callback functions are a fundamental concept in programming, particularly in JavaScript. They are functions that are passed as arguments to other functions, allowing for flexible and dynamic behavior in code execution. When a higher-order function is called, it can invoke the callback function at a later time, often in response to an event or after completing a task. This is especially useful in asynchronous programming, where operations like network requests or user interactions can occur without blocking the main thread. Understanding callback functions is essential for writing efficient and responsive applications.