fnServerData - what is aoData?

I have code:

$(document).ready( function() {
  $('#example').dataTable( {
    "bProcessing": true,
    "bServerSide": true,
    "sAjaxSource": "xhr.php",
    "fnServerData": function ( sSource, aoData, fnCallback, oSettings ) {
      oSettings.jqXHR = $.ajax( {
        "dataType": 'json',
        "type": "POST",
        "url": sSource,
        "data": aoData,
        "success": fnCallback
      } );
    }
  } );
} );

What is parameter aoData? How can I display values form aoData? I use alert(aoData) and I see: [Object object], Object object], Object object]...