The Hidden Debug System Plugin is a cutdown version of the default System Debug Plugin that comes with Joomla.
After installation, simply go in and disable the default System - Debug plugin within your Plugin Manager and leave just the Hidden Debug plugin enabled.
If you then enable Debugging in the Joomla Configuration page the hidden debug plugin will then start to add debug output within a hidden HTML comment at the bottom of each of your pages in the frontend.
This is useful for production sites where you can't really make full use of the debug plugin and you want to test something out (like, a caching plugin) without it displaying anything to your users that could potentially be bad to display with the normal debug plugin (for example database table name details).
This plugin just gives you Profiling Information (time elapsed for page to be generated), Memory Usage, and # of Database Queries executed.
As an optional feature you can enable email notifications to be sent out to you for pages that exceed a certain number of queries (this was useful for me to learn a bit more about the number of queries different pages on my site was generating and then I was able to turn it off).