addBack() - add the current selection to the previous one and merges those selections

parent() - selects the direct parent of the current selection

<div id="parent">
    <div id="child">
    </div>
</div>

console.log($('#child').parent()); // return the div#parent selection

console.log($('#child').addBack()); // return the div#child selection - it merges the div#child with the previous selection (which happens to be empty)