The values of form elements including type='hidden' are submitted to the server when the form is posted. input type="hidden" values are not visible in the page. Maintaining User IDs in hidden fields, for example, is one of the many uses.
SO uses a hidden field for the upvote click.
<input value="16293741" name="postId" type="hidden">
Using this value, the server-side script can store the upvote.