$pkey is a variable that contains the name of the form element
so for the form element named:
Registration_group_Email
<?php echo(str_replace("_"," ",$pkey)); ?>
removes the under scores and converts it to:
Registration group Email
to remove the fist part, add another str_replace to remove "Registration_group_"
<?php echo(str_replace("_"," ", str_replace("Registration_group_","",$pkey))); ?>
see this tutorial for details on removing some form elements from the email:
http://www.webassist.com/community/tutorials/view_tutorial.php?tid=21