A callback function is more-or-less an absoluteley normal function that 
processes the results of time-consuming code.

Basically, when performing time-consuming actions, you have two choices:

a) Performing the action synchronously:

The time consuming code will be run inside your main code. While doing this, 
everything is blocked (this includes the user interface (i.e. the website 
you are showing)). The user may think the website has crashed and therefore 
close it. 

b) Performing the action aynchronously:

You could compare it with a program using threads. You call a method that 
launches a thread. When launching, you give a reference to your 
*callback*function as a parameter. In this thread the time consuming action 
will be 
processed, while the frontend is completely usable. When the thread 
finishes, the *callback* function you gave before will be called with the 
results of the processing. The code may be a bit more complicated, but the 
advantages of asynchronous requests outweigh the additional work.

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