In Confluence 5.8 and earlier, a What's New popup appears the first time a user logs in to Confluence.   If you don't want to see this , you can turn off the What's New popup for your whole site:

1. Go to  > Add-ons
2. Search for the Confluence What's New system add-on (you'll need to select System or All add-ons from the filter drop down)
3. Disable the add-on. 

The What's New popup will not appear for users on first login, and will not be accessible from the Help menu. 

In Confluence 5.9 and later, the What's New popup has been removed, and the What's New item in the Help menu points to the release notes. Disabling the add-on, as described above, will hide the What's New item from the Help menu.