testfunction("test1","test2","test3","test4")


function testfunction(...testValues)
{
  var values:Array = new Array();
  values = testValues;

  trace(testValues[2]);
};

This allows a varying amount of parameters to be passed into a function.  Which is helpful if the amount of parameters passed in could change.
The trace in this example returns test3.

Pass in an varying number of parameters.

ActionScript 3