scrolly.js understanding a simple function code

scrolly.js understanding a simple function code

Hey guys , the below is a parallax scroll pluggin : a rather simple one : 

from line 71 - 77 : what are these lines of code really doing ?? (i hv pasted the code below)

  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.     };
i have really been struggling to get my head around this ! , Like really , what is it doing ? 

Thanks .