这几句jQuery代码看不懂，求解，谢谢！！！！！！！

<script type="text/javascript">


            (function($,sr){


                var debounce = function (func, threshold, execAsap) {


                    var timeout;


                    return function debounced () {


                        var obj = this, args = arguments;


                        function delayed () {


                            if (!execAsap)


                                func.apply(obj, args);


                            timeout = null;


                        };


                        if (timeout)


                            clearTimeout(timeout);


                        else if (execAsap)


                            func.apply(obj, args);


                        timeout = setTimeout(delayed, threshold || 100);


                    };


                }


                jQuery.fn[sr] = function(fn){  return fn ? this.bind('resize', debounce(fn)) : this.trigger(sr); };


            })(jQuery,'smartresize');
</script>

特别是这句，jQuery.fn[sr] = function(fn){  return fn ? this.bind('resize', debounce(fn)) : this.trigger(sr); };

jQuery.fn[sr] 是什么意思，谢谢！！