A callback handler is a programming construct that allows a function to be passed as an argument to another function. This enables the second function to execute the first function at a later time, often in response to an event or condition. Callback handlers are commonly used in asynchronous programming, where tasks may take time to complete, such as fetching data from a server. In many programming languages, such as JavaScript and Python, callback handlers help manage tasks like user interactions or network requests. By using callbacks, developers can create more efficient and responsive applications, as the program can continue running while waiting for the callback to be executed.