A callback is a function that you pass as an argument to another function, so that it can be executed later—often after an asynchronous operation completes. In asynchronous programming, callbacks are essential because they let you specify what should happen once a time-consuming task, like reading a file or waiting for a timer, is finished. This pattern helps you avoid stopping the entire program while waiting for something to complete.

Callbacks make it possible for your code to keep running without waiting for long tasks to finish. Instead of blocking the program, you hand off a callback function to be called once the asynchronous operation is done. This way, your program remains responsive and efficient, handling other work while waiting for results.