Web page-about the principle of invoking jquery Ajax interaction

Ajax Web Page javascrpitjqueryphp

For example, code like this

 $.ajax({                'url': '/chk_user.php',                'type': 'get',                'data': {                    'ajax': 'checkuser',                    'username': value                },                'cache': false,                'async': false,                'success': function(data) {                    redata = data;                }            });

View by browser developer mode
is actually visiting the http://www.xxx.com/chk_user.php?ajax=checkuser&username=123456&_=1430449833478 page.
What about this _=1430449833478 parameter? My direct access to this address will return an error, so I guess this last parameter is a random number or time to filter for illegal access.
So how did it come to be calculated?
Little brother looked at a half-day source code, really do not understand, trouble to know the guidance under, thank you

