It is getting a value indicating whether the page request is the result of a call back. Its a special postback, so a round-trip always occurs; however, unlike the classic postback, the script callback doesn’t redraw the whole page. ViewState is not updated during a callback, it is for postback. IsPostBack is true when a method in the code behind posts a page
Checks whether the Page is accessing the server for the first time or not. Unlike the IsCallBack, the ViewState is updated. IsPostBack is true when an AJax call, calls back a the page.
How to make a callback?
references:  Script Callback