It contains shortcuts to different parts of your site, along with notifications for comments and for when updates are available for plugins, themes and WordPress itself. Plugins and themes can also add custom links to the admin bar. For instance, if you have the localization plugin WPML installed, you can change your active editing language in the admin bar.
As default, the admin bar is fixed to the top of your window when you’re viewing your site while logged in. You can disable that feature by going to Users → Your Profile, unchecking the “Show toolbar when viewing site” checkbox and clicking the blue “Update Profile” button at the bottom of the page. When you have done so, the admin bar will only be displayed when you’re viewing the administration panel.