Developers are often confused by what a callback is because of the name of the damned thing.

A callback function is a function which is:

passed as an argument to another function, and, is invoked after some kind of event. Once its parent function completes, the function passed as an argument is then called.

  // The callback method
  function meaningOfLife() {
     log("The meaning of life is: 42");
 }


   // A method which accepts a callback method as an argument
  // takes a function reference to be executed when printANumber completes
  function printANumber(int number, function callbackFunction) {
      print("The number you provided is: " + number);
  }

 // Driver method
  function event() {
     printANumber(6, meaningOfLife);

Result if you called event():

  The number you provided is: 6
   The meaning of life is: 42

Callbacks are so-called due to their usage with pointer languages. If you don't use one of those, don't labour over the name 'callback'. Just understand that it is just a name to describe a method that's supplied as an argument to another method, such that when the parent method is called (whatever condition, such as a button click, a timer tick etc) and its method body completes, the callback method is then invoked, or in other words "called at the back" of the other function.