it's a jquery plugin that inverts the $.param function (http://api.jquery.com/jQuery.param/).
$.unparam(string)
http://jsfiddle.net/yeikos/9Sv3p/
<script type="text/javascript">
var str = 'a=one&b[]=two&b[]=three&b[c]=four';
console.log($.unparam(str)); // {"a":"one","b":{"0":"two","1":"three","c":"four"}}
</script>