A callback provides an asynchronous way of doing something. If you have a thing that needs to be done, and you don't want the application to sit around waiting for it to be done, then you assign the job to some other class, and when that class has done the request, it sends a callback to you. For instance, when you create a listener, you are assigning it the job of listening for events. Then you go on and do other stuff. When an event happens, the listener makes a callback to you saying "An event happened", now it is your job to handle the event in your callback method.
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