The 'published' setting for a page allows you to publish or unpublish that page. Only published pages are visible in the menu. This way, if you're still working on a page, and it's not quite ready yet, you can unpublish it and hide it from the menu. Anyone with the page's URL can still view the page, but they'd only know it existed if someone sent them the exact link.
Here's how to modify the published setting. First, visit the pages section of your Admin Control Panel. Then, click the page settings button.
Next, check or uncheck the "Published" checkbox. Finally, remember to click "Save!"