GlideAjax call to server side.

Hi, 

GlideAjax call to server side. 

When we use client script to call server script then we use 

line 1 ga.getXML(callBackFunction); 

line 2 function callBackFunction(response) { 

line 3 var answer = response.resonseXML.documentElement.getAttribute(answer); 

I have used it many times. 

Can anyone please make me understand line 1, line 2, line 3, why we use this line everytime in client script. 

Regards, 

Nivedita