In programming, a callback is a function that's passed as an argument to another function, and is intended to be executed after the main function has completed its operation. This mechanism is often used in asynchronous programming, where it helps to prevent blocking or waiting for an operation to finish before the program continues.
It's common in JavaScript, especially in Node.js, to handle operations that take time, such as network requests, file I/O operations, or any kind of event-driven, non-blocking operations.