The  nth-child selector selects all elements that are nth-child of its parent. The div:nth-child(2) will select the second <div> inside any container that has two or more direct <div> children. if you wanted to get at the fourth list item, In that case, the appropriate selector would be nth-child(4). The Jquery implementation of the nth selector is derived from CSS specification.

Syntax    :nth-child(n|even|odd|formula)

Example

<!doctype html>
	<html lang="en">
	<head>
	  <meta charset="utf-8">
	  <title>first-child demo</title>
	  <style>
	  div {
	    float: left;
	  }
	  span {
	    color: blue;
	  }
	  </style>
	  <script src="https://code.jquery.com/jquery-3.4.1.js"></script>
	</head>
	<body>
	<div>
	  <ul>
	    <li>Sibin</li>
	    <li>Jobby</li>
	    <li>Bobby</li>
	  </ul>
	</div>
	<div>
	  <ul>
	    <li>Sam</li>
	  </ul>
	</div>
	<div>
	  <ul>
	    <li>Kevin</li>
	    <li>Shorn</li>
	    <li>Ginu</li>
	    <li>Midhun</li>
  </ul>
</div>

<script>
$( "ul li:nth-child(3)" ).append( "<span> - 3rd!</span>" );
</script>
 
</body>
</html>