the $_GET variable collection refers to the querystring at the end of the page address.
the variable $_GET['id'] referes to a url variable named id, for example:
you can pass that along to another page using a link:
<a href="page2.php?id=<?php echo(isset($_GET['id'])?$_GET['id']:""); ?>">click here</a>
as long as the id querystring variable is passed as part of the page address to the email page, it can be used in the email.