Webpage-question about how jQueryajax interaction is called

For example, this code & quot; $. ajax ({& #39; url & #39 ;:& #39; chk_user.php & #39 ;,& #39; type & #39 ;:& #39; get & #39;, & #39; data & #39 ;:{& #39; ajax & #39 ;:& #39; checkuser & #39 ;, & #39; username & #39;: value}, & #39; cache & #39;: false, & #39; async & #39;: false, & #39; success & #39;: functio ajax webpage javascrpitjqueryphp

Such code

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

View In browser developer mode
Is it actually accessing the http://www.xxx.com/chk_user.php? Ajax = checkuser & username = 123456 & _ = 1430449833478 this webpage
So what is the _ = 1430449833478 parameter? When I access this address directly, an error is returned, so I guess the last parameter is a random number or time to filter out illegal access.
So how is it calculated?
After reading the source code for half a day, I really don't understand it. please give me some advice. thank you.