Admin bar the bar on the top of the page that contains important administration screen links such as add a new post, see pending comments, edit your profile, etc. Plugins and themes can add more functionalities to the admin bar.
After WordPress 3.1, the Admin bar was introduced. and now it is used as a navigation menu from preview pages and as a site location managing menu in Multisite Networks
Admin bar is activated by default in WordPress core installation.
But you can deactivate it from Users » Your Profile.