A hidden URL field is an anti-spam feature that we use on Wikia. It does not appear on the edit page, so any edits which add content to this hidden field are assumed to be bot-generated and will be rejected on save.
Why? If spammers try to save an URL on a normal edit, they are asked to complete a captcha, or the edit will be rejected because the URL was on the spam blacklist. If no URL is added, it is harder to block the edit.
However, Wikia receives many edits from spam bots where a piece of junk text is added to a page and no URL is saved. We believe the lack of URL may be because the spam bots are designed for blogs. On a blog comment, there are fields for your URL and for your comment. On sites without that URL field, such as wikis, the bots are leaving only the junk comment without any spam links. By adding an URL field which ordinary users can't see, we know that if that field is filled in, the editor is a spam bot.
Other ways we are dealing with this problem are to block commonly used junk phrases from either the text or the edit summary. (Before we blocked it, "How r u? nice site!" was placed on many pages across Wikia. On sites other than wikis, the phrase is accompanied by spam links.) However, the spam bots frequently change the phrases they use and sometimes use words that can not be blocked in this way since they are commonly used for genuine reasons (like "Thanks").