explain concept callback function on httpclient thx

i'm new about the titanium, below is the code teach by my leader today but i got something confusion, anyone can explain and clarify what i explain, and explain more explain to me thx~ it work with HTTPClient and callback function with pass the parameter.

what is the purpose of call back?

app.js
--------------
 
 
-var callFunction = require('gallery');  
 
var function GetString(jsonString)
{
    /*
    The jsonString will be return and pass something from the callback
    */
}
 
callFunction.get(GetString);

gallery.js
--------------
 
exports.get = getData;
 
function getData(callback)
{
    /*
    var str =JSON.parse(this.responseText);
    callback(str);
    */
}