Help with adding a jquery alert box after form is correctly submitted

[eluser]iwillsoarlikeaneagle[/eluser]
A question.

Code:

function(data) { // when your callback func. answers here we go
            // in this example your callback function returned variable "result"
            // in json encoded

            alert(data.message);

            // return a flag (from callback funct.) when the formdata is saved and everything is fine
            // print message here -> depending on the flag ("success" or "try again")
        }

What does the data parameter do in this function? Where does this come from?