$.ajax中async的作用

//当移动电话或者家庭电话不为空时才校验
   var valResult; 
   if (mode == "1" && IsJdjfEnableControl == "1") {
        MobileTel = appForm.MobileTel.getValue();
        HomeTel = appForm.HomeTel.getValue();
        if (MobileTel != "" || HomeTel != "") {
            $.ajax({
                url: ‘/Slxt/PUB/Pub_XMLHTTP.aspx?ywtype=CheckTelAndCardID‘,
                type: ‘POST‘,
                data: { MobileTel: MobileTel, HomeTel: HomeTel, CardID: ‘‘ },
                dataType: ‘HTML‘,
                async: false,
                timeout: 1000,
                error: function() { alert(‘操作失败，请关闭重试！‘); },
                success: function(result) {
                    valResult = result;
                }
            });
        }
    }