If you are referring to the form1_processor.php and form2_processor.php, they check that the POST data required is completed.
This code takes the selections from form1 and lets you pass to form2 if you selected the required fields. (form2 has a set of fields that are dynamic based on form1's choices.
IE, on my form1 you select the videos you used (for a teachers form), then on form2 it has you rate them and comment on them, so form2 needs the answers form form1 to build form2.
You could use Javascript validation on the forms, instead of the _processor.php pages, but I chose php to make my own error code (from NogDogs examples).
Does that help/make sense?
Don