scrolly.js code understanding

scrolly.js code understanding

hey guys i was just checking out this online pluggin , its basically a parallax plugin , a very simple one . 

can somebody tell me what this bit of code from line 70 to 77 is doing , the ocde is as follows : 

  1.  $.fn[pluginName] = function ( options ) {
  2.         return this.each(function () {
  3.             if (!$.data(this, 'plugin_' + pluginName)) {
  4.                 $.data(this, 'plugin_' + pluginName, new Plugin( this, options ));
  5.             }
  6.         });
  7.     };
and that syntex ,  $.fn[pluginName] is very unfamilier to me , sould't it be  $.fn. $.fn.pluginName ??? or are both the same . 

here is the link to the plugin code on git : github link .