Sometimes there are values that you want to pass along as part of the form that you want to give a predefined value to. Perhaps you have similar forms that can be submitted from more than one page and you want to identify the page that it was submitted from. Whatever the reason, it is simple to add extra fields to what is passed when the form is submitted by adding hidden fields.
I haven't included a sample page showing a form that only has a hidden field as such a form would make no sense whatever as there would be nothing to see. Instead we just add the appropriate input tag to define our hidden field anywhere in the form (immediately after the form tag is a good place to put any hidden fields).
This article written by Stephen Chapman, Felgall Pty Ltd.