[jQuery] ajax success event does not work

Hi,
Can anyone explain me what does success function arguments
contain..for ajax jquery..
I read the following in the documentation

success -> Function
A function to be called if the request succeeds. The function gets
passed two arguments: The data returned from the server, formatted
according to the 'dataType' parameter, and a string describing the
status. This is an Ajax Event.

function (data, textStatus) {
  // data could be xmlDoc, jsonObj, html, text, etc...
  this; // the options for this ajax request
}


"i want to know what is the data and textStatus"

I have this code here :

$(document).ready(function(){
$("form#regist").submit(function() {
                var str = $("#regist").serialize();
                $.ajax({
                type: "POST",
                url: "submit1.php",
                data: $("#regist").serialize(),
                dataType: "json",
                success: function(){
                        $("#loading").append("<h2>you are here</h2>");
                        }

        });
                return false;
        });
});

the success part does not work.. i can enter the data into the DB if
all fields entered are correct..However if something is wrong then
nothing happens.

Can anyone guide me where i am going wrong..Thanks a lot..