Makes the ID number of the current page or post visible in the Admin Bar.
If you are asking this question, you don't need this at all. The only time to bother with this plugin is after you've been frustrated a few times trying to track down an id.
The inclusion of id numbers on the "view" items in the Dashboard only works in WP 3.3 and up. If you are not seeing those numbers, then you may need to upgrade your WordPress installation.