Problem understanding return

Problem understanding return

guys check out this fiddle : 


its uses the filter function and the return statement , what i don't understand is the use of the return statement , whom is it returning what ?? 

The HTML :

  1. <ul>
      <li><strong>list</strong> item 1 - one strong tag</li>
      <li><strong>list</strong> item <strong>2</strong> -
        two <span>strong tags</span></li>
      <li>list item 3</li>
      <li>list item 4</li>
      <li>list item 5</li>
      <li>list item 6</li>
    </ul>
and the Jquery code 

$(document).ready(function(){
$( "li" )
  .filter(function( index ) {
    return $( "strong", this ).length === 1;
  })
  .css( "background-color", "red" );
});

i just don't get where the returning statement is returning to ...