中间的NUM：是啥意思，为什么要这么些

                $("#btnCheck").bind("click", function () {
                    $.post("http://www.imooc.com/data/check_f.php",{
                        num: $("#txtNumber").val()
                    },
                   
                    function (data) {
                        $("ul").append("<li> "+$("#txtNumber").val()
                        + data + " </b></li>");
                    });
                })